Yes. You can opt in and out on an individual basis. Once you opt in there is a delay (3 days) before you will be fully removed and no longer be open to attack from other VvV players. This is true whether you quit the guild or you quit VvV.
We have no plans to roll this out to other facets beyond Felucca. This was a question early on in the development process and when we weighed the pros and cons the cons outweighed the pros, so we decided to limit VvV interaction to Felucca. The traders are in both facets for the sake of convenience.
As far as artifacts are concerned, the reason they are usable in all facets is because we did not feel it was appropriate to force those who wanted to participate in VvV to have to change out gear every time they want to go and do Trammel based activities. The artifacts will not last forever, and participation will be required to acquire more of them.