1. I would like to protect my watch-only wallet with a pin or a password. (this option is greyed out) Now anybody who runs the app can see the amount of money in my wallet, I think it is private information which could have been protected from unauthorized viewing. The linux version has this protection why the android one doesn't?
Unfortunately i can't answer your question regarding the reason for a passlock not being chooseable.
But you can use apps which do restrict the access to applications with a pincode / password. There are quite a few available (named safe/vault/..).
Android does have an own app-safe from version 6+ which can be used to restrict access to applications.
Note that (on a rooted phone) your information can alway be seen by other applications when you are using/unlocking your wallet.
2. I can restore a BIP39 seed mnemonic on the linux version, why I cannot do the same on the android one?
Electrum does not support BIP39 in the first place. An additional option to use BIP39 is only given in the desktop version.
But there are other mobile wallets available (especially for android) which do support BIP39 (e.g. MyCelium, Coinomi, Samourai, ..).