1. so the wallet files are on the device - ok. But are the wallets also in the cloud somehow ? If not, how can this wallet sync (betwwen 2 devices) work ?
Electrum uses a deterministic wallet, so the private keys can be deterministically derived from your seed on each device. The wallet file carrying the seed (so the private keys) is kept in the device. Servers only give you the transactions history and broadcast transactions.
2. i can only have 1 wallet file with electrum on android ?
As of right now, you are right. If you want to have different wallet files in Android, you'll need to rename the electrum.dat to other name and rename the other wallet file to electrum.dat each time you want to switch wallet files (there's no multi-wallet support or wallet file switching in the current version).
3. a read-only mode on android is not possible - is it ?
i use the same wallet - on PC i want to use it as a real wallet - on android i just want to watch it ...
When creating the wallet in the Android phone, you have to enter the Master Public Key of your wallet that is in the PC. This will give you a seedless wallet (no private keys) in the phone, you'll be able to create new receiving addresses but you won't be able to spend from it.
A new version of Electrum for Android is being developed, with lots of features.
PS: Have you had problems installing Electrum 1.8.1 in Android? There's a known problem with the QR code scanner due to a broken SL4A and latest Android OS versions