1/ Booking codes are no longer required; enter your retailer (can be Other - so TrainSplit covered), date, origin and destination then select your train. This eliminates the issue you describe. I did post this is the seatfrog thread just after your previous post on the same issue:
https://www.railforums.co.uk/threads/vtec-and-seatfrog.164537/
2/ Just searched for LNER upgrades today and I see plenty, plenty, plenty.
3/ My own experience is really, really good. Had loads of upgrades from LNER and VTWC. Not tried with GWR yet and am unlikely to!
4/ For those of us without priv tickets or railcards covering LNER/VTEC/GWR it can be a really good deal. I'm mystified by the negativity on this forum for this app!
5/ The best thing for people to do is download the app and try it for themselves. You don't need a booking ref or ticket to try the app. Use the website to read the fine detail; e.g. use of first class lounges is granted but the statement is squirrelled away on the website and not mentioned in the app.