I want to wire it to be switchable between the 1:10 and 1:20 windings.
You only mention one cartridge. One ratio is going to work MUCH better than the other. If you're going to run a bunch of carts through it, then there is a convenience factor. If you're not rolling through cartridges, putting a switch on very low level signals is something that is worth avoiding.
On a related subject, have you determined done the math to determine what you need for your cartridge? I found
this guide very helpful when starting out with MC/SUT matching. I still frequently refer to it.
I plan to directly wire short lengths of blue jeans lc-1 cable with switchcraft rca connectors on the input side instead of female rca connectors. On the out side, I plan to use female rca connectors.
This is backwards from the "normal" way of doing this. I guess it works if your arm is terminated with female RCAs, but if you ever want to use it with an arm that is male terminated, you have to introduce a bunch of unnecessary connections. Male terminating the output is a reasonable option (although in my system I have found having a jack there better so I can run STP interconnects with the shield only grounded at the preamp end better from a hum perspective, but YMMV).
Ideally, I'd like to include some sort of option to try different resistances across the output leads.
This is nowhere near as necessary as people tend to think. It is a much better idea to find the right transformer ratio for the cartridge. Again I will refer you to
this guide for more information on this topic.
3. What the heck do you call the grounding terminal doohickey - you know, drill a hole in the project box and screw it on. A thumbscrew on the outside of the box for a ground wire and a soldering lug on the inside...I have searched Mouser, Digikey, and Partsexpress to no avail. I guess I just don't know what that thing is called!
It's called a binding post.