If a catholic who is already a priest marries, he can no longer serve as a priest except to hear a confession if a person is in danger of dying. I don't think they are permitted to say mass privately, although some do. I don't think any catholic who is married can be accepted in the seminary. This has been discussed at length on the internet.
A catholic can transfer to a rite that allows married priests. Ministers and priests from other churches who convert can be priests, (Anglican, etc.), but if their spouse dies, they are not allowed to remarry and continue in the priestly ministry.
Yes. A priest can marry civilly. Now, he should not receive the Eucharist, since he's married outside the Church. But, in the cases I cited, these guys (all of whom I know and knew in the seminary) were married for ten or even twenty years. Their marriages didn't work out, for one reason or another, and they ended in divorce.
Every one of them sought permission, through the local bishop, to return to the priesthood.
Rome granted every single one of them the right to return. Three are pastors, and two are associates and will likely be pastors within a year.
I understand that most priests who marry or long to marry do the right thing and leave the priesthood, so it usually doesn't come to this, but are you saying the Church has no recourse when a priest flaunts the rules and marries anyway?
In the case of these men, the Church did nothing. Now, if any of them had sought laicization (as many priests did under Paul VI and the early JP II pontificate), they would be considered laymen. I have heard of eight or ten cases of laicized priests who divorced, or whose wives died, who have petitioned Rome for reinstitution. I have no idea how any of these turned out. However, I know of a permanent deacon in our diocese whose wife died. He was 42 and had kids at home. He wanted to remarry, so he sought laicization.
Rome told him to forget about it, and remarry if he wished.