I noticed a couple of self-trust issues in your list, when you mentioned you were worried that you might not be able to cope with your emotions. And there is an "A-trust" issue, when you mentioned you worried she would leave you for P. So, a couple of trust issues going on there.
Some of this, you need time and experience to lighten some of the worry loads. So I'd say the wise way to proceed (into poly territory) would be really slowly. This way A has some time to figure out where she really stands with respect to this polyamory thing (and you have a chance to directly observe how she approaches that).
There is perhaps even a bit of limerence going on here, as you are experiencing the fear of the loss of a relationship that seems to be "bigger than life" (and I guess it could be). Another reason to go slow on the poly front, so that your emotions don't race ahead of you.
How does A feel about all this? Have the two of you talked much about it?
Love means never having to say, "Put down that meat cleaver!"