1 - Does anyone have any recommendations for a similar TYT radio that I should look to use as a 'closest' starting point? I've been looking at a few different TYT models and while the themes are similar there are some reasonably big differences among them. I'm not familiar enough with their product line to know which ones might be the closest to the TH-8600. If no one knows, I'll just suffer though figuring it out.
I can't answer this, but someone else probably can.
2- This radio supports a few features ( 2Tone & 5Tone signalling, etc.) that aren't typically used in amateur radio. Is it normal practice to fully tease out the entire memory map and add support for these types of things to CHIRP, or just skip them?
The answer to this is "whatever you want". Some people love nothing more than teasing out every last bit in the memory map. I am not one of those people ;) You kinda need to implement enough for it to be useful and pass tests, but it's not critical to implement everything. I'm guilty of usually just implementing the stuff in the radio that _I_ use until and unless someone asks for something specific.
--Dan