So do just about all the pistol buffer tubes accept/use a castle nut?  Whichever tube I go with I want to be able to use a castle nut and an IKH endplate.

No.  MANY of them do not use a castle nut.  if you want to use a castle nut pay attention to which one you pick.  I went with the Phase 5 not because I wanted to have a castle nut but because I wanted to use a QD end plate.

Which ones use a castle nut?  I also measured my AR carbine stocks and I like a pretty short LOP

This might be a dark gray area but I have seen quite a few pictures of AR pistols in this forum that "looked like" they had a carbine buffer tube wrapped in 550 cord.

Or you can do the modification and have that tube made round on a lathe (which does take a machine shop) or other ways.

Not a grey area at all, you can have a regular carbine reciever extension if you want to.  There is a tech letter in the stick at the top of this forum.

Would it be a bad idea to avoid the carbine buffer tube if you have a collapsible stock in your possession? Couldn't that be considered intent? Just wondering because I am building a pistol and I also have a regular rifle lower as well.

Of course you need a legal "useful purpose other than making an NFA firearm" for any group of parts in your possession, out and about, or even at home. Having an AR rifle or AR rifle  upper with you covers having a stock. It even covers having an extra stock, legally, though an extra unattached one obviously might not be the wisest thing to have out and about .. no sense baiting the tiger.

- OS

As long as you have a legal purpose to possess the stock e.g. a rifle capable of accepting it,  or an upper/barrel which when installed FIRST can be used to turn the pistol into a legal length rifle, you're good to go.

You have a rifle reciever,  you may possess extra stocks for said reciever.

I would remember to wrap the pistol's  buffer tube. Just to make sure you can't readily install a stock.

Also it makes a better handle and feels better on your skin on cold days.

Whether it might make a difference in perception or not, "readily" is not a condition, doesn't matter if carbine tube is wrapped. "attachable" is not defined in the SCOTUS ruling. Push come to shove, even a pistol tube is not necessarily an absolute defense -- tape could attach one, etc. Same applies to a vertical forward grip, too.

The best and simplest thing is to always make sure any part/component in your possession has a legal useful purpose.

For example, this kit I used to tote is perfectly legal (lower of course began life as handgun).  However, same assortment without a rifle component would be making an NFA firearm:



- OS