Humanity is frequently at risk when performing Rites:
Only check for Humanity gain from Banish if
Do Humanity Check/Gain rolls get the usual role-playing/story/tactic bonuses and if so how?
They can. However, the severity of an action does not apply as a penalty for a Humanity check -- depending on the game's definition of Humanity, being rude to someone might incur a check... so might killing, raping, and butchering them (in that or any other order), and each check requires the same type of roll. 
Is there any way to earn bonus dice on the Humanity Check portion of the rituals?
Yes, but it wouldn't be based on a rollover from victories on the previous ritual rolls; it would have to be based on some other action relevant to Humanity in a positive way that was also relevant to the ritual. 
When do you get a Humanity Gain roll?
When you perform the action that merits Humanity gain. Actions to enable you to take actions that merit Humanity gain do not count.