The answers are bellow:
1) litereplica executes every transaction to the master db and to the slave db
False. The SQL commands are executed only in the master database. Then it copies the modified db pages to the slave db and update it. So both dbs will be binary equivalent, byte by byte.
2) If I somehow 'forget' to attach the slave db and execute some transaction on the master db only, then the two db's are out of sync
Yes. The master db is updated and the slave one is out-of-date.
3) There is no mechanism to get these two databases in sync again.
False. As soon as the slave db is connected to the master db they start the synchronization process.
It is a one way sync, sending modified pages from the master db to the "slave" db.
Once all the modified pages are sent, they are considered in sync. And the slave/secondary db will receive new updates on each new transaction executed in the master db.
4) So - if I replace the master db with an older backup version I have to replace the slave db as well.
False. The litereplica will update the slave db with the db pages from the master db, and both will be binary equivalent. It does not matter which content is in the slave db file, it will be overwritten with the content from the master db. The slave db can even be empty (0 bytes) or not existing at all.
You can think of litereplica as an incremental backup system, that transfers only the modified db pages from the main db to the secondary ones.
There is no big problem if the secondary db is off-line (as long as the connection be reestablished when possible).