Now that SilencerCo’s latest innovation: the integrally suppressed Maxim 50 is out there, could one build it legally with out a form 1?
Second question is since muzzle loaders are not considered firearms by the ATF can one put a shoulder stock on a pistol or have a SB muzzle loader rifle?

The ATF has determined that a "silencer" on a non-firearm is not a silencer, if it is permanent. This was first applied, I believe, to airguns and paintball guns.
Your brace query should also be ok.
Your questions were all towards federal law so I'd suggest you also look into your local and state laws.
The reason I say this is because even if I have a legally approved form 1 for a 10" SBR'd AR15, in CT, the state sees ANY firearm w/ a barrel <12" to be a pistol. So, under CT law, depending on configuration, I could have an illegal pistol. While I don't recall anyone getting jammed-up for this, it's out there.

My main question was how to make the baffles/tube with out a form 1. I understand once attached permanently it is not considered a suppressor, It is the period of time between fabrication and attachment that I am concerned about.
You said you have a 10" SBR AR 15, does CT consider that a pistol or SBR?

I'm sure no one would know you were making it, but:
Make the tube and rear mount, first. (I'd weld or permanently affix via other means)
Mount suppressor to barrel and perm attach.
Make baffles and end cap (a monocore or welded stack can add further insulation to law)
Insert said baffles.
Attach end cap and perm attach if you didn't do monocore or welded stack. (I'd prefer to just weld it shut)

I say welded stack or monocore to relieve some doubt you were trying to make a part that could be used elsewhere. However, I'd prefer to have a sealed unit, perm attached to barrel. Just make sure you take cleaning into account. She gonna get dirty

Again, the ATF has said that a silencer isn't a firearm/silencer if it can't be readily adapted for use on a firearm, ie. a real gun.

More info in US, Appellee, v. MICHAEL CROOKER No. 07-1964 and ATF Rul. 2005-4

CT doesn't have a definition for "SBR" so that's where the barrel length comes in. There's even more of a process now (post Sandy Hook) so you don't have to worry (read as your screwed either way) too much about it anymore. We've recently resorted to building 12" "others" w/ braces, forward grips, and making sure of a 26" OAL. Not really defined as anything as it's not a rifle or pistol and not an AOW federally.

Enough to make your head spin