Here is a review, comparing the x1.4 vs x2.0.
When I was doing the info search, prior to buying this lens and converter, I seem to recall some reviewers saying that the 1.4 was sharper than the 2.0. I think the x1.4 would be a safer bet, and brighter too.
No, these adapters can only be used with very specific Olympus lenses since that have a deeper recess at the back end, and the adapter has a projection that fits in there. For example, they will not fit on the Olympus 12-100mm. Too bad about that, but that is the way they are designed, only for a few compatible lenses.
If you do a search on Youtube, there are several really well done reviews talking about using this lens, and well worth the time to check it out.