Looking at the Avanti seats pre and post refurbishment, the new ones don't look too comfortable! I haven't tried them myself yet, but I did like the old ones
First on that 397 does look good, it must be said, and I've not yet tried it but if the price is right, I'll be there!
Regarding your proposed trip in October, I think it's both a good idea and a bad idea. Good because it'll ease pressure going into the mock exams, in theory you'll then go into the week of mock exams more relaxed. Bad idea though because even though they're just mock exams, I would be using the time to get into the right mindset and prepare for them. I did nothing of the sort when I did GCSEs over 20 years ago, and if I had then I'd have done better than Cs and Ds. All easy to say with hindsight and knowing what came after secondary school!
Might I recommend postponing your next ALR until next year? I know it's an absolute age away yet, but there's a number of positives to waiting for it:
1) More time to save up for it, even with a railcard it won't be cheap!
2) Doing one so soon after your last ALR has the possibility of not having the same 'magic' to it. One's first ALR is always special, and to repeat it again only a few months or less later probably won't feel the same. Especially as your first one was in First Class, to then do another week of ALR action in Standard is going to feel incredibly different!
3) As productive as doing one in October will no doubt be, don't forget the daylight hours will be hugely reduced, so less to see out of the window!
4) On the same sort of note, the weather will be significantly different too, especially in terms of temperature. Unless of course we get a heatwave which is not too likely, possible of course!
5) My final thought on this is that if you were to choose to do your next one after the real exams, it would be quite the way to celebrate the achievement of finishing them all. It's a tense time leading up to and going through exams season, both for the student and for the parents, so having a way to release all that tension and celebrate the conquering of that time could be worth its weight in gold
Obviously, it's up to you at the end of the day, the above list is just some thoughts to consider first.