I think one needs to differentiate between GV and naturalisation.
If you want to live in Portugal for the duration of GV period (i.e., 5 years currently), then your full family needs to get a GV (or atleast a D7/D6 visa), which will count towards the time. And GV gives you one path to naturalisation.
Where you mention whether Parents having spent 5 years in Portugal is sufficient for an adult child to qualify, the answer is yes as per current laws, but that the child must show connection to Portugal as per the 4 criteria I mentioned above in my earlier post, which should be able to carve a path to citizenship outside of GV.
For the rest of the scenarios where the child does not turn 18 by the time of your naturalisation and you dont want to reside in Portugal, there is no need to get a GV for your child.
Parents above 65 (like with minor children) are considered dependents, and hence qualify to be tagged into the GV application. Again you need to differentiate between GV eligibility and naturalisation, as elderly will need to spend the necessary 35 days in Portugal to qualify for citizenship.