I will answer your questions, but let me start by saying the chances of you getting hsv1 from her ghsv1 are essentially zero. Ghsv1 rarely sheds, and rarely recurs, making transmission almost never happen.
1. Are there any other ways to reduce likelihood of transmission during intercourse beyond condoms/antivirals? For example, wearing briefs in addition to the condom to reduce skin contact
How long has she had ghsv1? If she's had it longer than 2 years, recent studies show that she is probably shedding 4 days a year. If it's a new infection, shedding rates are higher. We have no studies on what Valtrex or acyclovir does for ghsv1, but assuming it works the same way, your partner has to decide if taking a daily medication is worth it for her.
2. If I touch her genitals with my hands and then touch my genitals inadvertently is there a material risk of transmission?
Hands do not transmit any STD. There is too much involvement with air and movement to make hands a risk factor.
3. Are there any known cases of genital to genital HSV-1 transmission in a relationship abstaining from genital intercourse (or ~once every couple of months)
No and I've never heard of any transmission where people are actually having sex. Don't deprive yourselves of anything you want based on herpes. If she gets outbreaks, just avoid sex when she has outbreaks. Wait until they've healed.
4. If I were to semi-purposefully contract oral HSV-1 from her, would this then reduce my likelihood of getting it genitally? I have never had cold sores and tested negative recently
No, I wouldn't do this. You are more infectious from the mouth, and could get frequent cold sores. The chances of you getting it genitally are so small that this doesn't make a lot of sense. Also, the hsv1 IgG misses 30% of infections. You could already have it and not know it. I mean, it would work - you couldn't get ghsv1 from her this way, but you could end up suffering far more than you need to.
5. If I am wearing underwear and she 'grinds' on me to the point of abrasion on my genitals, is there a risk of transmission?
No, it doesn't go through clothing. No STD does.
I understand your concerns, but you are way overthinking this. She isn't a walking infection or biohazard. You can touch her, give her oral, have sex with her, enjoy her. If she's recently infected, the shedding rates might be higher, and taking daily suppression might be a good idea, but if she's had it more than 2 years, personally, I wouldn't bother unless she's getting outbreaks.
If you find you can't relax about this, let her go. She deserves someone who won't worry about every little touch, and you deserve to relax and enjoy sex.