You'll want some sort of waterproof membrane somewhere in the shower wall. Did you can put anything like 6-mil poly sheeting between the cement board and the wood framing? Or tar paper?

If not, you can remove the cement board, install the poly, then rehang the cement board. Easy to do if it's screwed up, not so easy if nailed.

If nailed, or if you just don't want to pull down the backer board, you can add a topical roll-on membrane on the face of the cement board. Something like RedGard or Hydroban or Mapei HPG.

You'll do the membrane after the cement board seams are thinsetted and mesh taped.

Yes, I put the membrane between the stud wall and the backerboard.

Excellent, I just thought I'd ask about the membrane to be on the safe side.

I normally do the rough plumbing and electrical prior to hanging the membrane and cement board. When done with the rough plumbing I'll end up with galvanized nipples sticking 3" or so into the shower where any plumbing will come through the tile; the location of the arm for the shower head, the location of the wall outlet for a handheld on a hose, the location of any body sprays.

The shower valve will also be roughed in, around it will usually be a "plaster ring" or a protective plastic shield provided by the manufacturer.

THEN I'll hang the membrane and cement board, cutting holes as needed for the cement board to fit around the nipples and valve plaster rings.

With access to the plumbing wall from behind, you can install the plumbing after the cement board is hung, but sometimes cutting holes in the cement board after it's up and fastening plumbing components to the framing can be a little more difficult. But not impossible.

So back to your questions:

Now I need to basicly tape and bed the seams, screws, corners with the mesh tape and thinset right? Yes, use mesh tape, not paper. Do NOT let the thinset build up on th ewall, you want the thinsetted seams to be flush with the face of the thinset. If you use too much thinset, you'll end up with humps at each joint.

Should I do the plumbing into the shower before tile? Yes. Have your rough nipples and the shower valve body installed prior to tiling.

How long after I tape and bed do I wait to start tiling? You can mesh tape and thinset the seams as you tile the wall. That can be a "safe way" as you wont end up with wall humps on the seams. If you chose to do them separately, do the seams Day One and tile Day Two.

How long do I need to wait to groute? You can usually grout on Day Three, the day after you tile. Read both the thinset and grout instructions though, some might recommend waiting longer.