You could see the floodlights of the old Reading FC ground from the up platform at Reading West.
I've never been to the "new" Reading stadium; I know it is close to the new Green Park station, but just how close are we talking about, I wonder: 5 - 10 minutes' walk, or less than that?
20 mins flat walk through clearly defined straight paths through a business park and a straight bit of road, maybe more for boozed up footy fans, hence why the bus is more worth it for locals (ten mins into town down the A33, maybe 15 mins at the worst of Reading congestion and then connecting buses onwards).
At £2 for a single or a return, I think the matchday buses are very cheaply priced already! Reading Buses are pretty demand responsive with these so if they get busier I expect that (subject to available vehicles) more journeys could be run.
The current Greenwave 50 service covers a large part of the Green Park development- it is true that some will switch to the train (likely if travelling on elsewhere such as London, Newbury or Oxford) by train but if just going to the town centre the bus will still be quicker so I can see this continuing at a good frequency even after the station opens. Kennet Island is closer to town than to Green Park station, now with its own separate shuttle bus service so these people are not likely to go out of their way to the new station. Likewise RIBP is a good distance from the new station and it will still be quicker to use the calls on the Mereoak Park & Ride which calls in there.
Yes Reading West was is not far from Elm Park site, and I expect there was time that trains did make additional calls there, but the station (even after the proposed enhancements) is highly unsuitable for mass volumes of passengers descending on it at once, so there would be serious safety issues against such calls. What's more, the most appropriate walking route is actually further than from Reading General.
I know, but prohibitively high parking prices for those who are not disabled/elderly could enable RFC to further subsidise the shuttle and increase public transport use by spectators.
The Greenwave services are a really good service for residents at the moment, and KI in particular, as you highlighted, will not lose its service as it is a trek to RGP station from there.
However, I know the rest of the network is heavily subsidised by Green Park/ RIBP themselves, and they might switch some or all of that cash to providing discounted season to and from the new station, especially as it will be faster from further afield.
Re Reading West, I'm aware of the circuitous walking route, that is what the jetpack joke was about

!
I am originally from Caversham, I used to do a few first aid duties with St John Ambulance at the Mad Stad.
I would suggest it would be faster from Theale than from RW, I'd like to see someone get through the water meadows/sewage farm/landfill and recycling/other industrial land between Coley and Madejski Stadium using any mode of transport.