i) how useful is it to just assign a genus?
Yes useful. If you also add something explaining why you cannot provide a more detailed ID that provides useful information to the observer as to what they need to do in order to get identifiable records. That sort of education is exactly what iSpot is for.
ii) how useful is a tentative ID?
The idea is that iSpot is self-correcting. If you put something down and people think it is wrong they can suggest an alternative. So yes add it, explain your reasoning, and let things go.
iii) does iSpot interlink with iRecord and, if so, should I exercise more caution?
I beleive some recording schemes do harvest records from iSpot, and some (more?) do the same from iRecord, However iSpot is really badly designed to be a recording scheme, its purposes is to help develop ID skills, so I'd always encourage people to submit their records directly to a recording scheme.
 For the avoidance of doubt this is not a criticism. iSpot is not meant to be a recording scheme so that it is badly designed to be one is simply a reflect of its different purpose.