Plumbing – use primer for multiple purposes

I would put on the baseboards, but only tack them in place so they can easily be removed when the drywall guy is done.

The reason he is probably asking you to install them, is so he can make sure you are happy with the finish before he leaves so he doesn't have to come back.

It's probably a good idea anyway, because it would be a shame to get all the painting done only to realize the baseboards don't fit right. Then you would have to start all over again.

The standard pressure test for new natural gas or propane pipe in my area (NW Oregon) is something anyone can do. Same for installing the gas line. (Why are there so many naysayers on a DIY forum? This is not rocket science:   Just a few new skills to learn.)

Before installing the gas pipe, I consulted with a pro (paid $80 for an hour of onsite questions). He had some good tidbits and some not so great. The best bit of advice was that to prevent leaks, make every connection with no more than two or three threads showing. Since there were 105 connections, this was a heavy duty workout over several days. Using yellow Teflon tape makes turning much easier—as does a 16 inch pipe wrench. The joint's seal is formed by the pipe and fitting being forced together and then settling (which takes hours to days). By turning until only 1 to 2 threads (out of 10) remain revealed, there was no leakage.

(The worst bit of "pro" advice was that he would have routed the pipe through the garage in the most obnoxious way. It would have been convenient to install, but a royal pain in the ass to live with afterwards. Instead, we opted to bore two holes through the garage foundation into the crawlspace and kept 98% of the pipe neatly out of the way.)

To leak test, obtain a $10 gauge as pictured (for black pipe) and install it somewhere on the pipe. In my case, I put it at the top of the riser pipe for the range. I explicitly ran 3/4 inch all the way to the range to suit the gauge, which has a nice benefit of smooth generous flow even when all the burners and ovens are on. (Total extra cost was less than $20 over 1/2 inch pipe, and no perceptible increase in hassle.)

Since every appliance has an independent cut off valve, I closed all those and the gas meter shutoff valve, used a bicycle tire pump to pressurize the system to 30 psi and waited. If the whole system does not hold pressure for hours and hours, there is a leak somewhere. At the end, the worst leak offender was the test gauge itself. I had to clean the needle valve and use gobs of Teflon tape to get it down to reasonable levels—on the tire valve thread into the tester body, on the gauge stem's thread into the body, and a great deal of tape for the gas pipe going into the tester. For preliminary tests I tried temporary caps at the appliance feeders and those were exceedingly leaky—the pipe caps don't seem to fit tapered pipe because they lacked enough thread depth. The valves on the appliance flex hoses didn't leak at all.

Once I was satisfied that 30 psi didn't leak significantly, taking five days to drop to 22 psi (demonstrably through the test gauge), I called for inspection. The mechanical inspector walked in, saw 22 psi and said "you pass" without looking at another thing. According to the test standards (see 406.4), it has to hold pressure for 10 minutes with no perceptible drop and he had no idea if I pumped it up just as he arrived or hours before. So much for inspection insuring safety.

For the nibco valve, does that mean that it would be able to withstand up to 600 PSI of (cold) air/gas pressure, but should only be used as a main shutoff for up to 5 PSI, and an appliance shutoff where the working pressure is less than 1/2 PSI?

Yes.

Should the other shutoff valves rated at 1/2 or 5 be able to withstand the 4 PSI of the pressure test?

Yes.

I plan to do the pressure test at about 4 PSI, with a gauge that reads up to 15 PSI. Should I cap off the lines before the shutoff valves (to avoid stressing valves, but not test them), or close the shutoff valves and remove the caps on the traps so that excess pressure leaking through valves would be vented instead of damaging the appliance regulators.

Why not be thorough? If there is a leak at 4 PSI, it will be very difficult to assess. Run the test at the maximum test gauge reading.

My house's regulator is set at 7" of water column, gauge pressure, and I need to test at a minimum of 3 PSI-gauge, and I'm located in Indiana, United States.

That is standard residential delivery, same as mine.