Audio in - splitters and External Mic vs Line?

I'm looking for a definitive guide to how to connect external (mono) mics to my Macbook Pro 13" (Late 2011). I have tried a handful of different splitters that supposedly take a TRS headphone and a TS (or TRS) mic, and bring them together into the TRRS jack on the Mac.

I have not found a consistent behavior that works. Sometimes the splitters don't accomplish anything. Sometimes they cause the Sound Preferences Panel to (arbitrarily?) show either External Mic (which works...yeah!!) or Line (which doesn't work, for some reason). I cannot figure out what order of connection makes it work consistently.

What I'm looking for are:

a) conductor paths that are necessary

b) anything about sequencing of connections that matters (i.e., some impedance needs to be recognized or something...)

c) splitters that work

I have tried splitters from

Alzo

Dukabel

Kingtop

I've also tried little adapters that supposedly translate TS or TRS to TRRS and none of those have worked either.

I have one component that does seem to work mostly consistently, and it came with a wireless mic package from Depusheng. I don't like the headset (too noisy) but the little splitter cable seems to work.

What's the trick here? Why don't the obvious solutions work?