Adam, PM me. I can probably help you in some ways.

A quick set of answers.
Heat treatment of the VZ24 is often not necessary, but that's the general rule, not gospel. I did one just last year and it came out perfectly. I can tell you how I did it if you contact me, but I write slow and make lots of mistakes so I'm not going to go into great detail here.

I installed an ER Shaw on a 9.3X62 Mauser 3 days ago.I use a lot of Green mountain barrels too, but they refuse to make the 9.3MM so they loose a lot of sales there. Anyway here is how the ER Shaw shoots in the very first try.

NECG parts are excellent. However they don't have exact copies of Rigby sights and so on. Also remember Rigby has been in business since the days of muzzleloaders, so their styling has changed a bit from era to era A set of sights from 1912 is not exactly the same as a set from 1940.
Here is what they looked like in 1860.

So before you can get down to the finer points you should settle on a particular rifle to copy from a certain era. Or at least select detail you like that were available from Rigby in a particular year.

I would go with checkering that is perfect for the wood you use. 18 LPI is far better then 24 LPI if the wood is a bit open-grained. Denser wood takes finer checkering. You make that decision when the gun is built and finished.