Hi u/mulletman13, I finally had a chance to try and repair my microwave today.

Unfortunately, none of the easily replaceable components were defective. All three interlock switches, as well as the slow-blow main fuse, were all intact. I put the meter on all four components and re-connected everything to make sure the connections were good, to no avail. I could clearly hear the relay clicking as well when I tried it without the cover on, so by process of elimination, it's the inverter or the magnetron.

I elected to buy a new microwave at this point.

Also, one of the secondary interlock switches is an open type, not closed, so if you were to replace it, you'd need to get one of these as well:

https://smile.amazon.com/Microwave-Switch-Replacement-KW3AT-16-Normally/dp/B083CYSJQW/

That said, if I understand how these switches work now, you could just get three of the dual-lead switches that u/HeadOfMax linked, which can be open or closed depending on if you use the top or bottom lead on the front, I think.

Also, the security screws can be opened with a flathead, but I have the iFixit Pro Tech toolkit, which has the correct hex-security bit in it. If you don't have one of these kits and you like to fix stuff, I highly recommend it. It has paid for itself several times over already.

Even though I wasn't able to fix it, I'm glad I bought the parts, came here for advice, and took it apart. At the very least, I know that I'm not throwing something away that could have been fixed for five dollars and an hour's worth of labor.