For the first question, yes. It is entirely possible to return to the lowest punishment tier. The number we know is "around 3 months of regular play per tier", which would imply 3/4 of a year to reset from a 14-day ban. But remember that's regular, positive play. Less than perfect play would take longer (months longer, even. Possibly 1-1.5 years).
For the second question, nobody knows. Well, Riot probably knows, but they deliberately don't tell anyone. They want people to be positive by default, not "I have 8 positive games left before I can flame again". It's probably several hundred positive games (without negative games) to go from "next is perma" to "next is 10-game restrict", and a substantially larger number of neutral games.