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Thats pretty much where i'm at. My trailbike has 175s and my park bike has 165s. Both bikes have very similar cockpit dimensions and I can't tell any difference in fatigue/pain from changing bikes. Wife has 170s on both of her bikes and shes 5'3 w 30" inseam.  On the topic of drivetrains, got confirmation that the box 9sd does not have the same pull as 10spd and is like other traditional 9s. That and also the fact they will want $625 for the 1 prime 9 kit makes it a no-go for anything from box. This does look kinda interesting though. Check out that cassette weight and 1to1 derailleur pull ratio😎 https://www.sram.com/en/sram/mountain/series/ex1?filters=&sort=Relevancy&page=1

Singlespeed would not make a difference. One could predict a good crank length based on some static measurements and a database of thousands of bike fits (I have a book with this kind of data), but a careful analysis of spinning legs would be more useful. Inseam is only part of the story because there is no predictable geometric relationship between where the bottom of your pelvis is and where your hips joint (greater trochanter) are. That's why I resorted to slowmo video and a borrowed carpentry laser.  Imagine putting 250mm cranks on your bike (and ignoring obvious pedal strike and toe overlap problems). You'd probably still set the saddle hieght the same height from the bottom of the pedal stroke, but think of the knee hinge required to turn it over at the top. Your knee will start to flare out and you'll feel that after some miles. If the ideal crank length for a rider's leg proportions is closer to 170 and they have 175s, it will be subtle and take a long time, but it might really start to bother the rider. Personal experience tells me it can make your riding miserable. Whatever crank length you're using, if your knees and hips don't hurt, don't over-think it.

I though an 8-speed chain should NOT work very well on a 9-speed cassette. I think you can move "down" (10s chain on a 9s cassette) but you might run into problems going the other way. Unless the chain is really mangled in a few places, you should be able to use it for a while by replacing broken links with quick links if you break a link. A basic chain "breaker" tool, chain wear gauge, and a quick link plier are good to have around. Replace the chain when you can confirm it's worn by measuring.

Get a chain checker, there are several of them. If you take it to the shop the answer will almost always be yes, and they will only sell the expensive ones. Go to amazon and get a chain checker and two chains. Having a spare hanging around is never a bad thing. All 3 will be the cost of an expensive chain from a shop. The key in replacing the chain is to count the links, not size it relative to the length of the old one because they stretch. If you just do the comparison you will end up with increasing longer chains and more problems.

I don't know anything about compatibility for 8/9 speed chains, but I know that I needed different master links for the SRAM 11spd, SRAM 12spd, and Shimano 10spd.  Regarding replacement of your chain, that should be done when it's worn out and not just because you broke a link. The bike shop can tell you when to replace the chain, or you can get a gauge that measures how stretched out it is. You should notice some degradation with your shifting when it is time to replace the chain.