How to make a Priming Manifold on top of the pool pump lid

You are going to need to "pig" the line. You will need to insert something very slightly smaller than the pipe diameter (so it doesn't get stuck!) into the pump side of the pipe (i.e. where the whiffle ball wants to go "to") a tennis ball or racquet ball might work, so might just some crumpled up news paper. Seal your pig into the pipe and rig up a fitting for the air compressor hose. Turn on the air compressor and your pig will travel to the blockage and then proceed to push it backwards out of the pipe.

You will probably spend $25-$50 USD on the fittings and such (depending on what you already own) but it is far cheaper than digging up the pipe and cutting it out.

Having done some plaster, but only on interior walls (not drywall, real plaster), and some finish concrete and masonry work, I have to say that the most useful thing you can do is build yourself some sample surfaces and try to learn there. Actually hardened materials are a major pain to remove, and will look awful if you do a poor job.

Remember that a great deal of the strength in the surface comes from the chemical reactions (including oxidation) in the materials. Therefore, you really need to have the moisture levels right. A big challenge when plastering is making sure that the underlying layer doesn't suck the water out of your mix, which will cause the layer you're putting on to fail (because it's too dry when setting up). Mist the surface carefully: too wet is just as bad as too dry. (Ask me how I know. Go ahead, ask ;-))

The remark from DMoore is also right: a smooth appearance over the area is quite important. Building some guides that allow you to check for high and low spots when you're still wet will make your life easier. I haven't done anything nearly as large as a pool, but this made my life much easier in large sections of wall that needed to be repaired in a historic building.

I watched the old-timey pros do some plaster over a brick wall in my last house, and it appears to me that the trick for seams is to have the assistant mixing batches to time thing just so. That way, the edge of the work you're just finishing is still fresh when the new mix shows up, and you never really have a definite seam (this is the "hot joins" bit you mention). Of course, to make this work you basically have to do the whole job in one go. Hard to do, if it's big enough. If you're doing this for the first time, I think planning to do this with only two of you will result in not being able to do that.

Do not neglect keeping the work clean as you go. You can make a pretty good job look like total crap by having stray bits of mess hanging around, and hardened Portland cement is very hard to clean up. (You can do it with polishing and so on, but it's way better to get as clean as close as possible in the first place.)

If you can find some smallish and nice finish-concrete or finish-plaster problem to try doing first, your hand and eye with the trowels will be way better when you get to this much bigger job. If you can learn to love wavy bits in your finished work, then learning as you go might be ok. But if you can try to learn in some smaller jobs first, I bet your life will be better. I wish I could go back in time and re-do the first ceiling re-plaster I did, because I could see how I got better as I went across the ceiling (I was out of time because my wife wanted me to cook something rather than butter the ceiling some more!).

Good luck.