Is a 6 Hour layover in Heathrow enough time to go see Windsor?

I don’t think you have enough time even though it’s not far. An hour to get out of the airport and an hour to get there and back plus security at Windsor and the airport is just too tight for my comfort.

It may be 40 minutes by train, but with two changes en route which introduces uncertainties.

All the buses to Windsor leave from Terminal 5, so if you were using any other terminal you would have to also transfer between terminals.

You may have to do that anyway between the two flights depending which terminals both use. Is this on one ticket, or a self transfer on different tickets?

Heathrow is always a gamble. Flight delays, weather, security alerts— so many variables at one of the world’s largest and busiest airports.
If your flight’s on time and your luggage is checked through all the way to Scotland, you might be able to get through Passport Control and take a taxi to Windsor and see the village and castle’s exterior.
If you’re lucky, you might have time to tour the inside. Time permitting, just take a 15-minute taxi to Windsor and buy castle tickets when you get there. Admission is every 30 minutes with morning slots usually available. Mid-day can get sold out. It’s recommended to allow one to two hours to visit the castle. I definitely wouldn’t mess with rideshares or buses or trains because they will take up to one hour each way. Time is tight and public transit/rideshares introduces too many pitfalls outside your control in unfamiliar territory—when you’ll likely be jet-lagged.
If you have to collect your luggage and take it through security all over again before boarding for Inverness, I would forego visiting Windsor.

Instead, visit Urquhart Castle on Loch Ness when you’re in Inverness.
Cheers!

Too many variables.

When is your trip?

Is it all on one ticket?

Which airlines are you flying?

Will you have checked luggage?

I’m just going to say NO. :-)

It’s incredibly tempting, I know, but… well, the others here gave you the reasons. So, TL;dr - NO. Sorry.

(Edit to say: remember what Rick always says! “Assume you will be back.” :-)

If you can use the passport E-gates to get through immigration, and are willing
to take a taxi and not public transport, and are willing to sacrifice the castle and
just stroll around town if time becomes tight, then I think you have time to do
something.

But as noted by others, there are risks. So I wouldn't buy tickets for anything
ahead of time, and decide what do to after you exit the secure area of the airport.
And take into account what everyone else has said regarding luggage, arriving
& departing terminals, etc.

OK. Thanks everyone, this does not sound promising.
However, I will fill in some blanks.
Too many variables.
When is your trip? September 6
Is it all on one ticket? Yes, Houston to Inverness with this stop in LHR
Which airlines are you flying? British Airways into Terminal D. Flight to INV is out of 5 as well.
Will you have checked luggage? Carry on, as per RST. Can bring it or get a locker.
And we have priority boarding.

Honestly, if we can't see the castle, we'll probably hang out in the lounge.

You should be back at LHR with at least 2 hours to spare before your flight to Inverness. Since your flight arrives at 12:20 PM, you have about 4 hours to clear passport control, get your luggage (if needed), store your luggage (if needed), hail a cab, do a fast drive by / viewing of Windsor, hail another cab and return to LHR by about 4:35 PM. Do you really want the stress? Find yourself one of the clubs in the airport, pay the fee and relax. Windsor to really enjoy takes about 3 hours or so. Not including the sites around it.

Keep in mind that you really have no idea when your flight from the U.S. will actually arrive. And when you get through customs/immigration is another huge variable. These sort of questions come up all the time on these forums, and a lot of it comes down to how flexible you are, and how stressed you want to be on vacation.

Thanks all. We'll find a lounge and explore the airport. Hopefully will be back to see the castle another time.

I'd suggest finding a lounge that has a shower facility. I did that one time on a long layover returning home and it made me feel like a new person to get all that travel yuck off. That would put you off on a good foot for your tour.