I think I pretty much have a handle on what goes into a 9mm conversion. Here's my plan, tell me where I might be going wrong:

- Rock River Arms 16" carbine barrel
- CMMG bolt
- Spikes Tactical magwell adapter
- modified Uzi mags
- H3 buffer (because I have one already)
- 9mm ejection port door
- 9mm gas deflector

I already have a spare A2 upper receiver laying around, so I'll use that. Is it necessary to block off the gas tube hole? Is there a good way to non-permanently do that? Could I run a bolt from inside the receiver and use a lock washer/nut on the outside?

I've read that seven quarters work well as spacers in the buffer tube. Is that a good starting point?

Should I get stronger hammer and trigger pins for the blowback action? If so, where should I get these?

BTW, a short barrel is not an option. It's definitely going to be 16".

Thanks!

No need to block the hole in the upper.

Standard hammer and trigger pins are fine to use with the CMMG bolt.

No need to block the hole in the upper.  I have two 9mm lowers one with a spacer and one with the quarter trick.  I forget which one I put the quarters in, as they both function 100%.  I can't give you any advice on the magblock or uzi mags, but a lot of people use those and like them a lot.  I preferred to have my bolt lock back on an empty mag and went with Hahn's dedicated and use metalform mags.
As for pins and springs, I use KNS anti rotational pins and they have been working well so far.

You should also check out YHM and X Products barrels.
A company called Amm Arms makes a good looking all metal magwell block for 90 bucks.
I've never had a single problem with metalform or ASC mags. ASC just seem a bit easier to load.
I've got the Hahn 9mm buffer in my and it works great.
I've got the standard AR brass deflector and have never had any problems with gas or bass coming at me.
The hole in the reciver is just fine.
You can get a 9mm spacer on eBay for next to nothing.
Just some things I've come across in my research and use on my 9.

Use a rounded AR15 hammer and KNS pins.

I  suggest a Spikes or RR 9mm buffer, your choice of H3 is to light
Why not get a dedicated lower CMMG or GPI can be had for $150- $200

There's nothing really wrong with what you've picked. But..

If you're going to get a 16" barrel go for a midlength and get the longer handguards.

Go with a JP Enterprises bolt. It's ready for single feed pistol mags and will still work with double feed magazines.

Modified UZI mags, why?  Colt SMG mags are basically the same thing but with a last round bolt hold open.