Hello and welcome to BW forums

It sounds like you did really well in the first part of the sleep training and were getting some good naps with LO going to sleep without you in the room - fantastic

I agree with Deb that it sounds like she needs a slightly longer A time time now. Being resistant to the nap and then waking after one cycle of sleep are both signs that LO is UT (under tired) for the nap.
She might not be ready for the full 3hrs which is usually needed for a consistent 2 nap day but on the shorter A time of 2hr 15 she isn't sleeping well.
I would increase the A time by 10-15 mins (say 2hr 30min) and see how you go for 3 days. There should be less resistance to sleep so it will be less confusing about when you should leave the room etc.
With the longer A time BT may need to move a little. If she does short naps you will still get at least 3 naps in the day. If 2 naps go longer then you will either need EBT (very early BT so the last A time doesn't get too long) or a CN and a later BT. This is a shorter term routine whilst she moves from 3 naps per day to 2 naps per day.
At 6 months (so very soon) most LOs have a 3hr A time and 2 naps, this could be coming soon for you.
when should I be removing my hand and/or leaving the room? When she is fully asleep (which is what I thought from reading the books) or when she is calm but not yet asleep?
Both of these are fine and used when LO has different needs. If LO needs you in the room you stay until she is fully asleep, with a hand on or shush/patting or just shushing, whatever part is needed. If LO is calm you can leave the room. You can always go back in and help more.
Keeping a hand on LO until she is fully asleep is not considered a prop because it is easily weaned. When this is happening frequently you can begin to do an on/off as she nods and falls to sleep and can reduce the weight of your hand when it is on (firmer and less firm), this way LO knows you are there, feels the comfort of you but doesn't get used to a constant weight/touch.
hope that helps. You can post here with your progress if you like.