I'm sorry this happened this way, but the chances of you getting anything are pretty low.
First, before answering your questions, her being on her period doesn't increase the risk. You didn't ask that, but I'm letting you know anyway. Herpes is not blood-borne.
1. What would be my risk of contracting HSV2 from this encounter?
We don't have exact stats for this, but we do know that men who are having sex with females in the context of a relationship have these chances of getting herpes:
Ghsv2 transmission, female to male, over the course of a year, assuming sex 2-3 times a week:
Only avoiding sex during an outbreak - 4-5%
Adding condoms OR daily suppression - 2-3%
Adding condoms AND daily suppression - 1-2%
Only avoiding sex during an outbreak results in about 1 transmission in 1000 episodes of unprotected sex.
2. How long need I wait until I get tested? Must I truly suffer the slings and arrows of my imaginative mind for 16 weeks before testing, in the absence of symptoms?
You can test now, with a type specific IgG blood test. If it's negative now, test again if you want at 4 months. I don't think you need to particularly worry about this, though. Most people will be positive by 6 weeks.
2. I am seeing another partner regularly, in an open relationship. Do I need to tell her about my exposure? How long should I wait until we have sex again?
Are you typically open about these things? I assume the open relationship is based on trust, so on that, I'd tell her. I don't think there is a huge risk here, but since there is even the slightest possibility, I think I'd want to know. If she were the one who had a possible exposure, would you want to know? (No right or wrong answer here - just things to think about. I don't know you, her, or your relationship to know the right answer for you.)
I can't say how long you should avoid sex. If someone is going to get symptoms, it usually happens within 2-20 days. If you have a pre-existing hsv1 infection (and about half the adult population does, and 90% of them never get symptoms so they don't know it), you may not get symptoms. Some people without hsv1 never get symptoms. You can decide to wait a few weeks to see if you get symptoms, or decide to wait for 4 months (yeah, I wouldn't, probably, but this is up to you), or you can discuss it with your partner and see how she feels. Maybe she'd be okay with having sex with condoms.
So this is long - but essentially, the risk is low, you can test now, again at 6 weeks or 4 months. Make sure it's a type specific IgG (the IgM for herpes is useless, don't let them do that test), and go from there.