AFAIK there is no requirement for a dependent to be necessarily on a dependent visa (H4). Dependent can be legally in the country on their own visa. In my case my wife was on H1B when we applied. She shifted on EAD after we got it in 2012. So I believe you can go forward with 485 filing with her on F1.
OTOH - F1 is a Non-Intent visa - that is - you have to show that you do not intend to immigrate (as opposed to H1B where intent is fine). That might be the reason why the first attorney might have suggested changing to H4. Prudent thing to do would be to go ahead with 485 filing with her on F1 - but not do things like visa renewal or overseas travel until you have EAD/AP in hand.