2CI+MDMA: great combo, the only thing some people don't like is adding the stimulant effects of MDMA to the stimulant effects of 2CI (a lot of people find the stimulation from 2CI to be a bit uncomfortable, but some people don't mind it - never bothered me much). Other than that subjective issue, great combo. If you're taking pressed pills, make sure it's not a speedbomb - I don't think the combo would be dangerous but it might be unpleasantly stimulating.
2CI is generally thought of as a milder psychedelic, but everyone reacts a little differently. 2CE is considered by most to be the most intensely psychedelic of the 2Cs. 2CI is generally known to have less of the 'mindfuck' aspect, and more stimulation and euphoria, compared to 2CE.
I've never heard of nitrous while rolling being dangerous. If there is a risk, it's a very rare issue. I love nitrous while rolling or tripping. It's pretty much the only time I have much desire to use nitrous.
There are no known health risks to taking 2CI on a frequent basis, but it's not super well studied yet so that doesn't mean it's necessarily as safe as, say, mushrooms or cannabis.
I went through a phase of a month or two where I tripped on 2CI and/or 2CE about 3 times a week, sometimes 2 or 3 nights in a row. I noticed tolerance building up, especially to the mental aspect of the trip (by the third night of 2CI binging it was mostly just mildly euphoric stimulation and visuals). After taking a break of at least a week the tolerance seemed to reset more or less completely. I haven't noticed any lingering symptoms or aftereffects.