3mm is ridiculously small. Did they say why they could only advance you so little given that you have a 6mm overjet?? A BSSO of 3mm is essentially pointless, get a genio instead if your chin bothers you
The 2nd ortho may well be right. Orthos seem to be hit and miss - I saw 3 orthos and 2 dentists - all but one ortho told me to get surgery. The one that didn't proposed an orthodontic plan that would have never worked, I guess she wanted the business for herself. These are the kind of orthos whose opinions you cannot trust. The fact that your ortho is reluctant to extract teeth makes me think that they are decent orthos whose opinions are worth listening too
Any idea of why you have such a large overjet when your back teeth are OK? What is the orthodontic plan for improving the overjet if you don't have surgery? It is very rare, I think, to have a class 1 molar relationship and yet have a 6mm overjet?
Which country are you in? If you're in UK, try and consult Paul Johnson? A number of European surgeons also to virtual consults