Welcome to the forum.
Given his test result, it is very unlikely your partner has HIV. And your negative test result at 6 weeks shows for sure you weren't infected. Despite frequent advice to wait 3 months for reliable results, it really isn't necessary. To your questions:
1) No risk. You weren't infected.
2) 100% reliable, or very close to 100%.
3) No, I have never seen a negative result at 6 weeks become positive later. It probably never happens.
4) From a risk assessment standpoing, you don't need any more tests. But you are free to have additional tests if it will help convince you that you weren't infected.
Here is a thread that discusses why you see frequent advice about testing at 3 months or later, even though it isn't necessary:
http://www.medhelp.org/posts/show/1347755
You needn't worry about this event. You weren't infected.
Regards-- HHH, MD