It will be tight getting back in 30 minutes, Adams Park is a good three miles from High Wycombe station and as said earlier because the ground is at a dead end at the top of a long street which also happens to be an industrial estate then it can be a pain in the arse trying to get out in a vehicle. Taxis might be reluctant to come to the stadium because turning around at the top is tricky when it is busy, so you might be best served by walking down to the roundabout at the bottom of the street and trying to get a taxi from there. I don't think you have any chance of making it back to the station if you walk or get the bus.
As for the train trip? Like I said, I work for the company who run trains between Birmingham and High Wycombe. There will be an inspector on the train, often they aren't booked to get onto the train until Banbury but there will be one. If you are on the "wrong" train then you can hope the inspector will use their discretion and let you off, especially if you explain you bought the ticket in good faith thinking you would be back for the 17:30 (might be worth telling them you missed the 17:30 by a few seconds!) but it really is up to them. There are some people who will let you off, there are some who wont. Probably worth mentioning that a lot of the inspectors on Chiltern are bluenoses though.
The_Rev, do you have any insight into travel to/from London (Marleybone)? I had thought I'd just get a return which could be used at any time that day. Do Chiltern require you to purchase onto a specific train?
Mary le bone
Can't help with ticketing though, I have absolutely no idea about that side of the railway. I'd assume you can get some kind of out of boundary ticket if you are coming from London and have an Oyster card, from Birmingham I'd just say check the website.
A day return from London to Wycombe is £18.00 which you can pay on the day. This ticket allows you to catch any train. You can not use Oyster Cards to Wycombe. A day return from Birmingham to Wycombe is £22.50, again you can purchase on the day and use any train. If traveling from Birmingham New Street you need to change at Banbury.