1.) Yes- back off the knurled ring, and adjust the eye piece by turning it. On a scope of 2.5X, you likely won't have to go far. Note- you want to get the reticle in focus, not the image. The image will be fine once the reticle is focused for your eyes. Great scope for shooting with both eyes open, btw. I even find myself doing it at 24X.
2.) It is not by any means a long range/target scope. It was designed for close-in deer woods hunting- fast acquisition at near, to very moderate ranges. I'd say sight it in the way it suits you best. IIRC, that difference between the top of the post and the cross-wire is a deer's brisket at 100 yds (18")
3.) I'm not sure on. I have a K-4 here that isn't- but that means little. If it has been nitrogen filled, you'll have to use a fair bit of force once the eye bell moves past it's intended range of adjustment. Parallax is more than likely set at 100. For a 2.5X scope mounted in proper (Low as possible) height rings, parallax should not be noticeable enough to be an issue. You're more likely to cheek the rifle the same way every time with a low over the bore scope...
Just as an aside, I'd avoid the period correct Weaver rings- they're a p.i.t.a. to get the cross wire level on. Spend a few bucks on burris all-steel rings- much easier to work with, without the aluminum bottom half of the Weavers....