Always better to give us the pictures first, then we can see exactly what you're trying to achieve. Given you've altered the far end of the waste run, and it's accessible, slotting a Tee into the run shouldn't have been difficult. Ideally you could have used a solvent weld tee, 45° bend to get you upright, then a piece of pipe and a 90° bend to bring it out to suit basin trap height. As The Novice has said above, make sure that bath waste isn't running uphill.
Rad, again put some pics up of what you have, and we'll be better able to advise. Valves may be tight from not being used for several years, WD40 isn't likely to help, and biggest concern is going to be are they likely to start weeping if disturbed....