Firstly, you are incorrect on the point about spare stock on Saturdays (and throughout weekends, for that matter). Stock needs downtime for maintenance, and a lot of stock which is “outberthed” away from main depots for weekday morning peaks’ traffic is returned at weekends for maintenance. I’ve been working with units in the past week which haven’t seen more than basic train cleaning and prep for several days, and they will physically only have been able to get to the maintenance depot yesterday or today for urgent repairs. If you’re going to postpone these efforts, or alter them, it leads to a lot more work, and in some cases it all turns out to be impossible. The press will go and take a photo of a yard full of trains from a vantage point; what they don’t see is that the person wandering between each train with an orange rucksack is actually doing door repairs, PA transformer replacements, aircon resets, you name it.
Crews do not equate to trains either - you need different crew for each service and it all needs to be matched up. It’s no good GTR having a spare 313 if only a driver and OBS are willing to do overtime - the company may be running minus a 377 for traffic, which the OBS could work, but instead it’s a 313 and no conductor is available. The cover conductor has been sent to Southampton but now you have a 313 they need to take to Seaford because there’s a big crowd wanting Lewes and Newhaven. Good luck getting that arranged in a reasonable time. (This is why most of the extra trains could only really run up the Brighton Mainline on Pride night - most available crew would have been able to work the route and traction, plus you can also work stock close to a depot or stabling point without running out of crew hours).
Preston Park could perhaps be used - but it raises more questions than answers. Platform 3 is OK but Platforms 1 and 2 are a relatively small island. It’s hard, if not impossible, to plan reasonable secondary escape routes from the station in the event of an emergency. Staffing levels aren’t brilliant and any extra staff would have to be put up in a room which is barely larger than a stationery cupboard. You also have very narrow, inaccessible entrance stairs, which are not suitable if a wheelchair user or even somewhere with a pushchair turns up. The entrance passageway is also long and narrow.
East and West Coastway trains can be just as busy as the mainline ones during such events. The West Coastway in particular becomes “full and standing” at the drop of a hat, and East is bad if you can only resource 4 or 8 coach trains.
Day Returns - this is already done. A large number of operators will use preset plans where “tickets for x are now valid the next day” during major disruption; GTR included.