If you scan your negs anyway, some colour cross over can easily corrected in post processing, that's why you don't see much difference between room temperature processing and processing at correct temperature of 38°C/100°F. For many scenes colour cross over won't show up even if you wet print. It's amazing how far off a process can be and the results still look ok to us.
My experience with printing paper (both B&W and colour paper) is that when it is long expired it does not only become foggy, it also loses contrast. Judging contrast of C-41 negs may be quite difficult, because C-41 is already a low contrast process, and the orange mask does not exactly help either. If you are 100% sure you have lower than normal contrast, extending CD time is the way to go. Underdevelopment means you lose shadow detail, whereas overdevelopment will increase grain, so it's up to you in which direction you are more willing to err.
About foam: some C-41 colour developers contain a wetting agent, which will show up as foam when you pour out the dev. Unless it's way out of control I wouldn't worry about it.