I'm trying to understand how DoT is affected by critical hits/buffs.

1) Are DoT skills affected by critical hits and damage buffs? (yes?)
2) Are DoT ticks individual, affected by whatever buffs are present at the time of the tick, or are they all set out from the initial cast of the DoT?

Here's a few scenarios:
3) 30 sec DoT cast on 100HP mob. DoT does 10 damage every 5 sec. Total = 60 dmg. 4) Base crit rate = 0%. I now use a buff which increases my crit rate to 25% for 15 seconds, and do exactly the same DoT cast. Does the critical rate apply for only the first 15 seconds, or the entire 30 seconds?
5) I cast the same dot with the same base 0% crit rate, but after 15 seconds I cast my 25% crit rate buff without interacting any further with the mob. Does this affect the last 15 seconds of my already existent DoT?
6) Assuming I swap crit rate for damage increase buff, are scenarios 4 and 5 affected in exactly the same way as they are for crit rate?