Everything else you mentioned is irrelevant, we are talking about all cells being charged at the same time which is really the only way that makes sense. There is no point in waiting to charge cells.
there are a number of ways to pluck the chicken. smaller more numerous cells -> same power, faster charging due to concurrency.
point was if your cable can't supply the power the entire point is irrelevant.
Not really, because smaller cells -> less capacity -> lower current for same C rating.
One 1000 mAh battery charging at 5C means 5A charging current, while two 500 mAh batteries in parallel leads to 2 * 2.5A = 5A.
And AFAIK typically a larger battery scales better, that is, less dead space for connectors/housing etc so more mAh per volume.
my understanding was it was the chemistry of the cells that impacts the charging rate, not the physical size. happy to be wrong on that though.
scaling has mean dimensions. whats best for energy density may not necessarily be best for charging rates.