1. Will the expiration of the UN arms embargo see Iran get lots of advanced weaponry from China?

Probably not.

China has had the ability to sell advanced systems both before and during the embargo. It didn't do so.

2. Take a look at @SIPRIorg's register of Chinese arms transfers to Iran. You'll see lots of SAM systems and anti-ship missiles, but no major transfers for nearly a decade, and although Iran has "discussed" buying fighter aircraft many times, the last delivery was in 1996!

3. The whole gambit around snapback needs to be judged in accordance with 3 facts:

1: China does not seem keen transfer offensive systems to Iran

2: Other deterrents will remain in place, like US designations of Iranian entities

3: Iran has a large domestic defense industry

4. Snapback proponents are exaggerating the risk that Iran will gain significant new conventional capabilities if the embargo is allowed to expire as per the provisions of the JCPOA. This is to hide the real aim of snapback: torpedo the credibility of UNSC resolutions.

5. The stronger argument for the embargo arises from the prospect of Russian arms sales, but even the transferred Russian systems don't give Iran any kind of superior offensive capabilities.

7. The Trump administration is damaging American diplomatic credibility over a largely invented threat, using an invented legal justification.

Are we supposed to believe it is worth it?

8. Also, see this piece by @jonathandfulton. https://www.atlanticcouncil.org/blogs/iransource/will-china-become-a-major-arms-supplier-to-iran/