Ask vs acks is just a regional pronunciation variant, nothing inherently wrong with it. I have noticed that a lot of people from backgrounds with enforced language uniformity (french and Russian speakers for example) have trouble with the idea though.

Less vs fewer.

Very few people say "acks" instead of "ask". It's not a common error at all. I would say "who"/"whom" is the only truly common one you listed, though screwing up "than"/"then" is at least common enough that one runs into it with some regularity.