You can use xqrepack to repack the latest chinese firmware or just flash the above global firmware; ssh access will be permanently enabled even after hard reset. You'll have to repack new releases, of course, and flash again if Xiaomi ever releases an update.
No, don't update any opkg packages yourself, you'll most likely break something, specially so if you update any base package.
To properly install adblock, you have to install the libustream-mbedtls package and change the fetch utility to curl. Also, run curl on the terminal (to see if curl is working properly) and make a symlink between the missing libraries, if there's any.