senior rail cards in UK -- worth it?

Yes it's worth it for the Cal Sleeper alone. The Railcard costs £35.

On a typical night (3 May) Inverness to London costs £350 in a Club Cabin, £305 in a Classic Cabin and £80 seated.
With a railcard those fares are £233.10/£203.10/£53.25

So your £35 railcard saves you over £100 on that journey alone.

Unless Stafford to Edinburgh is before 0930 on a weekday then you would be better off buying a Two Together Railcard for £35 rather than two Senior Railcards at £35 each.

At this current time you can buy discounted rail tickets before having the railcard, although that is scheduled to change sometime later in the year.

You can always buy Stafford to Edinburgh tickets on the day. The most you will ever pay, travelling off peak is £110.30 or £73.45 with a railcard. You would be very unlucky to ever pay that much as Advance Tickets should always be available at the last minute.
For instance to travel tomorrow I can see fares as low as £50 before railcard discount (£33.25 with railcard).

Stafford to Edinburgh isn't a local/regional fare- it is an inter city journey of 268 miles.

I don't think you have to have the senior rail cards before buying tickets at that rate; you just have to have the rail cards when you use the tickets. Last year I bought several tickets for long-distance trips in the UK before leaving home, then purchased my rail card upon arrival in London. That purchase, as well as an earlier one in Edinburgh, was easily accomplished.

You might consider the Two Together rail card as an alternative if you are absolutely certain you will always be traveling together. It's the same price as the senior rail card, but you would only need one Two Together card. There's a weekday prohibition on using the Two Together card during rush hour, so you need to be a bit more certain of your travel plans with that card.

If the Stafford you are referring to is located between Stoke-on-Trent and Birmingham, I suspect even buying two senior rail cards would pay off. Stafford-Edinburgh is not a cheap ticket itself, and then you have the longer Inverness-London trip. I don't think the rail cards yield any discount on sleeping-car accommodations, but I've never had reason to check on that. [Looks like I was wrong about that; Stuart has given you an accurate answer.]

UK rail tickets bought close to the time of travel can be among the most expensive in Europe, per mile. Considerable money can usually be saved by buying tickets ahead of time; the earlier you buy, the more you are likely to save, because those special-deal Advance Fares eventually sell out, and the prices seem to rise even before they sell out completely. I just took a casual look at fares from Inverness to London (not specifically for the Caledonian Sleeper) and found a range of 69 to 233 GBP. I used ScotRail.co.uk since it sometimes finds price reductions by splitting tickets (though you don't have to make extra transfers).

You should take a look at how ticket prices decline as you select travel dates farther into the future. The Advance fares are not refundable. I think you can change the departure time/date at a cost of 10 GBP plus the current fare differential. Therefore, you have to balance your (un)certainty about your itinerary against the probably substantial extra cost of playing it by ear. A rail card will at least absorb 1/3 of any extra cost you incur if you choose to wait.

I suspect sleeping accommodations on the Caledonian Sleeper sell out very, very early. Assuming you don't want to sit up in a regular seat all night long, you probably can't wait very long to buy those tickets.

This is very helpful, thank you. I like the idea of one "Two Together Railcard" rather than two senior railcards. The 35pound railcard would save 117 pounds just on the Cal Sleeper. Any additional trips such as the Stafford to Edinburgh would be even more savings.

To reserve the Caledonia Sleeper, I must pay the ticket price now. I think because I'm outside the UK (in USA) I can't purchase the railcard. So do I purchase at full price and get a refund later? Thanks for any help.

You can purchase the Railcard now, while still in the USA. As well as Stuart's timely and excellent advice, you can read more about this and how to do it, on the Man in Seat 61 website. Buy it on line, upload a picture, and there you go. One tip, the website for purchasing the card is a little clunky and it may take overnight to complete the process. But well worth the effort and the cost savings.

Assuming I'm on the correct, official website (www.sleeper.scot), there's a place to show railcards on the initial screen where you indicate your origin, destination and travel date. I wasn't sure whether I needed to say there was one Two-Together rail card or two (since the card would cover both passengers), but it didn't matter: Adding either one or two rail cards didn't change the cost of tickets in sleeper accommodations; it only changed the fare for regular seats. I don't know what's going on there.

That's interesting- From the Cal Sleeper about Railcard availability-
Senior Railcard – Up to a third discount on Club Solo or Classic Solo rooms and seated products
HM Forces Railcard – Up to a third discount on Club Solo or Classic Solo rooms and seated products
Veterans Railcard – Up to a third discount on Club Solo or Classic Solo rooms and seated products
Family & Friends Railcard – Up to a third discount for adults and up to 81% for children on seated products.
Disabled Adult and Child Railcards – Up to a third discount for the Railcard holder and a companion travelling together in either Club or Classic, shared or solo rooms and seated products.
Two Together – Up to a third discount on seated products.

So on the sample 3 May date two people without railcards in a Club Room pay £455, with Senior Railcards two people each in their own Club Solo room pay a total of £512.80 (if you select 2 guests in 2 rooms), 2 people, 2 Senior Railcards.

but if I travel on my own in a Club Room with a senior railcard I pay £233.10, which means that if you make 2 separate bookings for Senior Railcard holders each in a Solo Club Room you pay £466.20 (which is less than £512.80). That's complicated.

that's not very good. That's taking the proverbial.

I've known about various Two Together time restrictions, never seen such discrimination relative to the Senior Railcard.

But would you and your husband want Solo rooms?

I bought our two senior rail card from the US online at the National Railcard website.
Here is the website:

https://www.railcard.co.uk/senior/

I had to upload a photo of me and my wife for our separate applications. If you can take a selfie, it's easy.
And provide a passport number.
After you purchase the two tickets online. You will need to download the RailCard app onto your smartphone.
They email you a link to download the digital rail card that goes to the Railcard app on your phone. And that's what you show to any conductor who asks.

I was able to book train tickets on the national rail website with the senior ticket prices from the US. The website did not ask me for my senior rail card number.
But I think that may have changed. Try a dry run on the national rail website and see if they ask.

Is it a hassle? Up to you whether the time spent online is worth the savings.

And yes, I think you should buy the senior rail card now before you purchase the regional/local tickets before you leave the US.
Then after your senior rail card arrives, then buy the rail tickets.
Buying in advance gets a cheaper price than just showing up on the day to buy a ticket.

But you are committing yourself to a set time to catch the train in advance.

That's what I have done and I have all my rail tickets for our UK travel in May.
When you purchase rail tickets online, get the e-ticket option. They email it to you and you can download them and print it out and also load it in your Apple Wallet. We printed both a hard copy and put them on our iPhones just in case.

Thank you all so much! Going to pursue rail cards today!