b) Correct. IFR is different 30 minutes either side is acceptable, any different and it has to be changed or delayed. If an IFR plan is not activated within 30 minutes of EOBT then the IFPS computer dumps it and you have to re-file (so DLA's are important in those cases).
(Thought I posted this a couple of days ago but it didn't appear.)
Couple of points:
IFPS now requests DLA or refiling for changes in EOBT of 15 minutes or more. See the
IFPS Users Manual
That said, I don't think FPLs do get dumped from the system after 15 (or even 30) mins. If there's no slot, I've always found considerable flexibility.
The ICAO limits come from PANS-RAC (now PANS-ATM), for which my oldish version says (II 8.2.1.2)
In the event of a delay of 30 mins in excess of the EOBT for a controlled flight or one hour for an uncontrolled flight for which a FPL has been submitted, the flight plan should be [delayed or refiled]
So if you pitch up on Ground 35 mins after your EOBT, ATC has the right to have thrown your strip in the bin, but generally speaking they don't.