My understanding is that you must choose to be celibate or not prior to being a priest. And you must be married before become a 'married priest'... i.e. you can't marry once you're a priest. And, if your wife dies, you can't re-marry... I don't know why.
Ahh, well: yes, you must chose whether or not you will be married/celibate before ordination; canonically, before one's ordination to the Subdeaconate (enforced strictly in the GOA, while in the OCA and AOA they are dispensing with the requirement for subdeacons for awhile and allowing them to marry before becoming deacons).
As for re-marriage: if the wife dies, then the husband, in the case of the priest, is supposed to carry on as a widower, which is seen as a holy state to be in; the rest of the story I think Anna and pravoslavbob cover quite well. Dating members of the flock, of the spiritual family, is seen as a big no-no... and when it does happen, it causes quite a stir (I know of a few cases of priests dating around, and it gets ugly).
As for the later question, of leaving the Priesthood and then coming back, the Priesthood is seen as a permanent state; if one were to leave the ministry, they would be forfeiting their rights to practice forever, but unless they did something within a specific range of circumstances, their ordinations are still valid.
Let me explain better: ordination is permanent, if the conditions are met. If it is found out that the ordainee lied about something, or there was some circumstance unknown at the time of ordination that would have prevented the ordination, then ordination is seen as void from the beginning (while any legitimate acts done, like weddings and baptisms, are upheld by economy). Otherwise, if a priest commits adultery, murder, or anything else to get themselves "defrocked" or "returned to the status of a layman" or whatever it will be called, his ordination is seen as permanent, but overridden/nullified in practice by the synod, and thus, they can never practice again. If one were to "leave" the priesthood, they would be renouncing the office, and thus wouldn't be allowed to practice again.