Jacy,
I'd list as soon as you know what flights you are using .. it can't but help.

At T5 you check-in at the self service machines the same as commercial pax. If there are seats openly available at that time then you will be accepted and given seat numbers otherwise you will be checked-in to standby ... in eother case you then go to a baggage drop desk to get your bags tagged.

If you have been seated then your bags will be sent down the chutes and then you can go airside.

If on standby then you hold on to your tagged bags and wait for the flight to be closed at about STD -40. At closeout you need to go to what you called the 'non-rev' room. In there you will see screens for each flight that is due to depart and on that screen a list of all the standby PNR codes and if you have been onloaded, not onloaded or occasionally that you need to See Agent ( normally if its a jumpseat or group split ) and then follow the instructions given on the screen ... normally to go to the manned check-in area next door to collect you boarding pass and get you bags sent down the chute.
You then need to go straight to airside because if you are not through security by conformance time ( STD -30 ) you will be offloaded.
If you have supplied a mobile number in your listing you will get a text at closeout to say if you are on or not and then you go to the room.
Its generally not daunting and quite a slick process.

Although its a fantastic building theres not a lot to do while waiting at T5. Certainly no non-rev lounge ( if only ).
You can easily get to Windsor in less that an hour by direct bus from T5 .. you can find timings and details here ...http://www.travelinesoutheast.org.uk
if time is more constrained you could get a pub lunch and one of the small villages around Heathrow most of which you can get to by bus