I’m pretty sure there are other exceptions

other people’s voting paperwork

Same way as they would for the ID, They don’t. That’s not a common problem. Only people in the same household could possibly access the paperwork and as there are only 250-ish people voting at the same polling station, risk is high that someone voting twice with paperwork of a family member would immediately be found. That there are enough cases of this to sway election results is highly unlikely.

voting several times

Check list. Everyone that is registered at that polling station is ticked off on the list.

IDs are not routinely checked when voting in Germany at least. But there’s no voter registration necessary because everyone is registered with their municipality and automatically gets their voting paperwork sent home. You just need that paperwork to vote, no ID. I wouldn’t change that setup because giving poll workers ways to refuse voters is not a good thing.

Btw. I’ve never had to wait in line for more than a minute to vote. Voting booths are usually in walking distance (might be different in very rural areas) and each accommodates maybe 250 people for the day.

Also, there are polling stations in prison. The right to vote can’t be taken away.