How do you keep hair-thin piping straight when gluing it up?

On sequence, build the runs as separate sticks matched to each flange pair, don't try to form one continuous network. Dry fit everything against the skid with the runs marked in pencil, bend each run off the model, then install one run at a time working from the back of the skid forward. In-place bending is exactly why your hands block the joints, you're doing the forming in the worst possible space. Only join runs at the model when two lines meet at a tee, and even then I'll often join those off the skid on a jig drawn on paper and drop the little assembly in as a unit.

Yeah that back-to-front install order is exactly how I did my last skid build. I'd add that I mark the bend locations on the brass with a fine marker against the drawing first, then make each bend over a piece of dowel held in a vice so the curve stays even instead of kinking. Also for the tiny bolts, I touch a bit of blutack to the end of a toothpick to pick them up, place them on a dot of glue, and the blutack releases clean once it's set. Saved me from losing half a bag to the carpet.

I am running into a diferent problem with the connections between the lines and the mounting brackets. The pipe diameter is okay, but the fittings themselves seem to stick out too much even when I try to hide them with paint. I was hoping to make the transition look more seamless, but the brass sections seem realy thick at the joints. Does anyone have a trick for minimizing the visual bulk where the run meets the flange? I just want the piping to look like it's attached to the skid rather than just sitting on top of it.

The kinking thing, annealing changed everything for me. I was building a little compressor skid last year and gave up on brass for a while untill someone told me to heat the tube to a dull red and let it cool. After that it bends like putty, no waves at all. I just do a section at a time with a small torch and bend it over a dowel.

For the bolts, I use a dab of beeswax on the tip of a needle file to pick them up off a white plate, way easier than trying to grab them with tweezers. They stick just long enought to drop onto a glue dot.

For the bolts, work over a dark towel or a cookie sheet. I learned that after losing a whole bag of 0.5mm nuts into the carpet one evening. I also do the picking with a toothpick dipped in thick PVA, let it go tacky first, the bolt sticks just enough to move it, then a touch of CA wicks in and it's set. Takes the shake out of my hands too since I'm not gripping hard with tweezers. On the wavy bends, one thing that worked for me was packing the tube with fine copper wire instead of solder, stiffer support through the curve. Anyone tried the frozen water trick? I keep hearing about it but never got around to it.