Train from Edinburgh to York - advice please.

Why are you using a third party website, such as Trainsplit? If you look at the LNER website for 19 May, the 1230 LNER for 4 people is £94.40, the 1330 LNER is £80.80 for 4 people, both fares before railcards.
22 May is the Friday before the Bank Holiday, so no one except Grand Central has yet released tickets- you are too early. Network Rail have not yet authorised timetable release. It isn't clear why Grand Central are selling tickets for trains which may be rescheduled.

However the previous Friday, 15 May, 4 people on the LNER trains at 1227, 1232 and 1331 trains York to London cost a total of £84, so cheaper than your 22 May GC price of £128.

XC in particular regularly reverse the direction of travel of their trains, so it is totally impossible to know which side of the train any specific seats will be on.

These are the GC seating plans, which depends on whether a Class 180 or 221 is on your service- https://www.grandcentralrail.com/travelling-with-gc/on-board/seating-plan
From York normally it is a 180, but a 221 could be substituted.

Book a LNER train on their own website and you can choose your seats from a plan, but sometimes trains can end up running in reverse formation, so there is no absolute certainty of direction or side of travel.

Thanks so much isn31c! Yes - I was following the advice on the Seat61 site. Just jumped on the sites being recommended, right away, and had yet to go directly to the LNER site. Obviously not the right move.
I’m trying to time things for the best fares, on my days of travel, and I understand that too far in advance can be a negative.
I am a bit hesitant to pull the trigger on the train tickets as (2) in our party need vacation time approval and seems the best/advance fares are non-refundable.
We have secured our plane tickets with points and I have hotel reservations- but all things I can cancel if need be. But I don’t even want to put that out into the universe. :)
Thanks again!

It's worth saying that so called Advance tickets are always available until two minutes before travel, on that route, not at as low a price, but still a good price.
For instance if is now almost midnight but every middle of the day train tomorrow has Advance fares Edinburgh to York for under £50 each.
But there is one train which you won't find on the split ticket websites, due to how they work, which is £28.50 each, at just after 12 noon. You have to know that is the really cheap train, then manually split the fares. It is a time/cost trade off as it takes over an hour longer to reach York, hence why the websites can't find it.
That is pre railcard fare.

That GC train tomorrow for 4 people is £145.60, the LNER 7 minutes before it is £164.40 for 4 people. The GC Class 180 trains are quite tired now, but comfort wise are superior to me to LNER.
The 1431 LNER tomorrow York to London is £118 for 4 people and it isn't the only one at that price.

Although it sounds as if you won't be doing enough train travel to justify a rail card, I want to clarify that with the Two Together Rail Card, you only buy one card for two people.

Thanks for the continued input and acraven - you are so right! Maybe it would end up providing a savings. Thank you for bringing this to my attention!