1. I tried to cut out one of the existing metal nail on boxes used with K&T. One side had a groove for the L&P to sit in for support. I tried a sawzall and multitool, but think an angle grinder would of been better, whats the best way?
This is a fairly uncommon style of wall case / lathe interface, and it is just plain hard to work with when retrofitting. That lathe rail made for a more stable install, back in the day, but removal, without disturbing the existing plaster takes a lot of patience and time. You call the wall case a "nail on", but I have found the lathe-rail type mounted with a flat bracket nailed to the stud face is common, too. Bottom line, each opening is a new archeological dig. . . you never know what the original installer did until you excavate, and the faster you go, the more likely you are to tear something up.
Sacrificing cutting edges is part of the overhead of this work, and using sharp cutting edges is absolutely desirable at certain points, like when the wood lathe or backer board is being cut or trimmed, or when a stretch of steel must be cut.
Vibration and wiggling of the actual plaster leads to micro-cracks, then cracks, then failure. As Buck says, scoring the plaster can help. Remember, plaster on wood lathe is built up of three layers, generally, so scoring through the hard surface (the "finish" layer) to the grainier, softer "brown" and "scratch" coats, will help control the inevitable cracking to be where you want it (mostly).
All of this goes to say that using power tools is a hard art. . . the fact that the cutting edge will not instantly stop moving is the really destructive thing in the instance of an "Oh, no!"
As Buck says, dust control is a big deal. Lead in paint can be a difficult reality. Any power tool will have a fan that blows dust all over. . . hand cutting allows it to fall more directly down. I fashion a "pocket" out of a full sheet of news paper folded and taped to the wall immediately under the cut to catch the detritus. I make it a point to use "Blue" painter's tape for this.
2. To cut in a new single gang old work box, I used a grinder, worked OK with a masonary blade, but would a saw blade be better?
After verifying there is no lead on the surface, I will still cut by hand almost always. . .
3. For a round cut in box, the plaster ruined my 3.5" hole saw, any suggestions
The premium hardened cutting edge will go the furthest, but they all succumb to the rock that is sand and gravel. Diamond edges are great, but expensive. I settle for good "bi-metal" edges and swear by Lennox. . . individual recommendations will vary. Hole saws are really hard to use, in my opinion, with plaster, as they are soft steels, by comparison. They are available with diamond edges, however.
4. The luminares use Pan Boxes fastened to a 1x between two studs. The pan boxes work well with the 1/2" L&P, for a single run of 14-2. I would rather remove the pan box and put in a 3-0 cut in. Any suggestions which is best
At present there are extremely few hardware options that provide the NEC required weight handling capability other than the selection of paddlefan rated boxes. I saw my first 50# rated 4" steel octagon on an adjustable bracket at the BigBoxStore just a few weeks ago.
The 1x backer board for the old flat pan will be problematic, as the lathe will have been nailed to it for support. The fact that you can work from above helps immensely. The key to a good install will be a DEEP box. . . to handle the plaster + lathe + backer board depth. Most brackets are "helpful" by guiding the install from the joist face, but for this kind of depth, the bracket legs will have to be turned out of the way or even cut off to permit the box to settle deep enough to get flush, or near flush, with the ceiling finish surface. Don't forget that there are "round" extension rings, like those made by Arlington Industries, that can help bridge a large setback.
For cutting in a new round ceiling box, I get my location, then break out the plaster to the edges, and expose the lathe. With the backerboard also visible, I will then use a reciprocating saw with a good sharp edge to slowly do the cut. The backerboard will provide a lot of vibration damping solidity that will save the plaster, in this situation. If there is no backerboard, I will hand cut the lathe with a sharp blade. This makes dust and requires a large tarp, and if there is lead, should include a "Zip Wall" containment and a HEPA vac.
I am doing all the lighting with 14 NM from above, has a nice high attic. Receptacles will be from the crawl.
Previous HO had run a spider web of NM, I am using two home runs of 14 NM to 4 Sq NM boxes, kind of like a backbone down the center of the house.
Personally, I try my best to avoid J-boxes with blank covers, and do my splices behind light fixtures and devices. And when there are blank j-boxes by previous installers, I like to work the alterations to remove them, when practical, but this is personal goal. . .