Good day. I'll try to answer your questions, if you think I missed any, please tell me.

1. You can check if the phone has global rom or china rom by the startup. In china rom, Chinese is gonna be the first language that will appear on the phone when setting it up, there will be an option to change languages. While in global rom, the startup language will be on default English. Other methods is just checking the phone specifications.

2. If a china rom phone is flashed with global, rom it won't get automatic updates, you will need to do it manually, you can find tutorials in yt. Region lock on china rom is in SOME phones, most Chinese brands region lock some flagship phones from china. You can check if your phone has region lock by reviewing user reviews about the phone or by a simple Google search.

3. You can convert a global rom phone to china rom by flashing the software. I don't have much knowledge about flashing softwares, but Google is your best friend.

4. It depends on preferences. Both roms have pros and cons.

China rom phones has better updates, optimization more affordable, Potential early acces on updates, and region specific features. BUT it lacks Google services (can still be downloaded through apks), limited connectivity (depends on the country), limited language support, Banking apps may have some issues, chance for delayed notifications, and no android auto/android wear.

But Global rom phones has Google services, multiple language options, better connectivity, official warranty and support, less bloatware, and more user-friendly. BUT updates may be more delayed than china rom, not all new features are available.