Will a smart interval take longer if your MPA on the previous interval went deeper than the target MPA?
Yes, if the interval is based on a Target MPA duration and it is a rest interval.
As I thought, thanks. Explains why my workout went longer than expected tonight. Next question: if a smart trainer is in ERG mode, how is it possible to deepen the MPA beyond the target MPA?
Only through variations in power even though you’re in Erg mode. You’ll need to switch to Resistance mode if you really wish to dive deeper that what the workout has defined.