check this: InviZible Pro: increase your security, protect you, thoughts (android)

(not JS, but should help with mobile)

no

it happens if you change files on each device while offline, if you change then sync it is ok, if you change same file on multiple devices, that is a conflict as it can’t decide which file you want to keep

quick vpn over tor and firewall https://f-droid.org/en/packages/pan.alexander.tordnscrypt.stable/

So it is not good? I mean, looks like CDBC (government crypto) will come into play anyway, but it lacks privacy and anonymity.

What would you do if you are forced to use crypto?

I guess for barcode to connect to devices (device key sharing), it is optional, if you share something with other people (family)

each folder has its own permissions and allowed devices

you can add your devices manually and those are auto detected on LAN, you just need to confirm device key

proton mail requires another email or telephone number for verification

that is the best advice, but some are unwilling to change so at least give them some protection

fraud