Virtually all data is rounded, that does not make it non-interval is that was what was meant.
The answer to the question depends. If the variance is the same in the subgroups (or close) then you can. If they are not then there can be signficant issues in calculations.
Since regression has very similar assumptions to ANOVA I immagine this might be a problem with that as well, although I have never seen this issue brought up.
You seem to have two different dependent variables, the volumes and differences between them? Are you running both as dependent variables?