Train from London to York

Advance tickets are inflexible and non refundable. If you miss your train, you've lost your money and you'll have to buy new rickets. There is some tolerance if your train is cancelled or delayed, you can catch an alternative departure so long as it is close to the one you have missed.
Is there a reason why you would have to wait until 7pm if you missed the 2:30pm departure. Trains are very frequent, roughly every 30 minutes. Are you looking at trains that terminate only at York? Because trains going further north, to places like Edinburgh, Newcastle, Sunderland for example all stop at York.
In the UK, trains are classified using their terminus, "the 1233 Newcastle train" for example, but will stop at a number of station on its way.
Rather than book a return, book two single, one for the trip out another for the trip back.
I can see departures for York on the 7th every half hour or so costing between £56 to £102 in total for the two of you. If you were to buy one now between £66 to an eyewatering £229.
Could you not stay the night in London, or book the later train, stash your bags and have a look around at the sights.
Ways to save money, are to travel outside the peak hours, buy a Two Together Railcard for £35 that will reduce your fares by 33%.

Of course no one can predict late arrivals, delays at Passport Control, or rail/tube disruptions on the day. However, assuming on time arrival, normal Passport Control staffing (and you will already have your ETA), and normal transportation operations, 4 1 /2 hours should be ample time to get to Kings Cross and board your train.

The transfer with the fewest changes would be to take the Picadilly line, which stops at Kings Cross. I noted from your user page that you have some mobility issues. Getting wheelchair assistance (book with your airline via their "Manage your Reservation" window) from the plane to the Tube station will make that long hike not only easier, but faster. So I think a 2:30 pm train is certainly doable, all else being equal.

A quick look seems to indicate that an advance single booked now for a month from now is significantly cheaper than one booked for tomorrow. I will let our UK train experts weigh in on their assessment

My inclination would be to wait until getting to Kings Cross to buy and my choice of train would be the slightly slower (by 8 minutes) 2.33pm train, (still an off peak train) because it is a different (and far superior) type of train to the normal LNER trains.

The last off peak is the 3pm train.

You can only get a refund on your flexible tickets until 2359 the previous night. However when you get to Kings Cross if you have missed the last Off Peak you go to the ticket office and ask the agent to 'excess' your ticket to an Anytime ticket (or an Advance (train specific) if there happens to be one still available)- that is to charge you the difference in fare. Then you use the two tickets in tandem with each other.
So effectively yes you do get the credit, just a different way of doing it.

Then choose seats on whichever train you are on, using the LNER app, until up to 10 minutes before the train is due to depart.

On a flexible ticket you can use an earlier train if you happen to get to KX sooner.

Buy a Two Together Railcard and apply that 33% discount when you buy the tickets. If you have to 'excess' the same discount will apply- https://www.twotogether-railcard.co.uk/

I should have included for you to look at www.nationalrail.co.uk for booking your train.