This should answer everything.

As #Canker notes, Avid hydraulic brakes (Juciy & Elixir) have a bad rep, either you get a set which never go wrong, or always do. 2014 on brakes seem (so far) to be much better

If you are looking for reliable hydraulic brakes, current Shimano are fit and forget till pad changes.

For your questions

1 - As long as you don't damage the hose/rotor, you should have no issues in using them for long duration's

2 - For Avid, they use DOT fluid which isn't to use as nice as Mineral Oil, you first time will take the longest, from talking to other users, it can be hit and miss with them for servicing.

3 - Have been using discs for 10+ years now, and never experience full failure, used the previous generation of Shimano for a few years, and when below zero, the levers would get stiff, just needed a lot more pressure to get them to work