Brownie, I have a semi-copy of the H&A which I had made into a long range .45 rifle. �I can comment about a couple of things -  
1. �I think your barrel's twist is too slow for long range. �18" is the convention. �
2. �15/16ths is a pretty light diameter for this rifle. �As a target rifle, this is pretty light in my opinion. �But it may be what you need if you are trying to stay �under the 10# limit and depending on how you set up your rifle (underrib, thimbles and ramrod - or not).
3. �Your nipple's path should have a stop to protect it's threads - the size of the passage from that restriction into the powder chamber is nto something I can comment on, but you need to have it reduced from the 1/4" hole that the nipple fits into.
4. �You need to locate a source for platninum lined nipples. �Anything else burns out very fast with heavy bullets.
5. �Without commenting on the threading dimensions, be very sure that your nipple is as far in front of the breech plug as possible. �With the H&A hammer it is quite possible that the plug may be so long that you cannot get the nipple in ahead of it and still have the hammer reach out to hit it. �If the nipple's flash hole is right at the face of the plug, you may have fouling issues leading to misfires or hangfires. �So, yes, you want a short plug. �How short I don't know. �
6. �A dovetail works fine for a front sight. �I use one on mine with a Lyman 17A globe sight.
7. �cantilevered rear sights may work but your rifle will be rather light and recoiling hard. �Be careful that you don't end up wearing that cantilever in your cheek or eye. �I prefer a folding rear sight and have adapted 4 different sets of sights for mine. �
i. �Patridge style barrel sight
ii. Lyman or Marbles tang sight from a Rem model 14
iii. Scope blocks for external adjustable scopes like Feckers and Lyman STSs
iv. Goodwin style tang sight for long range
 
I know I haven't answered all your questions.  I don't have any qualifications to comment about thread requirements, but I hope this helps. �
 
Brent

With a 24" barrel, you will have a humdinger of a hunting rifle.  Mine is 18" twist and it hunts superbly, but does other things as well.  18" is overkill for just hunting though.  
 
Sounds like you have a plan, but when you get back to the .45 long range rifle, be careful about the steel used in the breech plug and the nipple issue because your pressures will be much higher than round ball or even hunting type bullet rifles.  
 
You will also want to think about stock shape for long range.  Mine has too much drop for that purpose to be honest.  I may restock it someday but I have other things to shoot long range now.
 
Brent