speeder - there are many different types of autism. the problem with the majority of studies carried out is that they tend to treat autism as one thing. and it isn't. it is a whole range of things under an umbrella term.
as you point out, some children develop entirely typically, and then suddenly and catastrophically regress. others may lose skills slowly, with a regression happening over a longer period. others still have symptoms present (in hindsight) from birth.
there is, of course, no way of knowing which category (if any) your child may fall into until after the fact.
there is good evidence for a genetic link in some types of autism. and strong evidence for latent-and-environmentally-triggered-at-some-point links for other types of autism.
my dd1 was autistic before she had her mmr (but she had a shitload of jabs - not all of them necessary imo, and several given without consent - when she was a newborn as we lived abroad). but the mmr did damage her gut to some extent, and it has taken many years to even get her back on an even keel with that side of things (she is doing well now). my dd2 is in a prime category for latent-but-could-be-triggered, and so has not had any jabs (yet). dc3, due next month, will also have delayed jabs.