I dont think the length of time has anything to do with the strengh of his feelings - my h had an affair for a year and although i know he thought for a while it was love it was never true love as i was always on the scence.
I do believe that men can see it as just sex - as long as ow was willing to be the bit on the side then why would he change things??
You are right he did have every opotunity to leave you but he didnt and now faced with the choice he is staying with you - that means a lot more than what she is offering.
I do agree though dont contact her - she will tell you lies and will relish in making you feel awful. she may well say he is only staying for the children ect.... dont give her the chance.
I think too dont worry what he wants - what is it that you WANT now - only you can tell if he is making enough of an effort to satisfy you. Make sure you are in control now and take the lead as to what you want - dont let him tell you when you should be over it ect.
I totally agree with the no contact stance - whatever he says please stand by your guns on this one - he may well say he 'owes' her a face to face let down - remember he owes her nothing - she knew he was a liar remember so she can jolly well lump it now.
He too is probably only telling you bits of the truth, again it is up to you how far you want to push for the whole truth.. it may be an idea to work through this with a professional.
Dont make any promises to him just yet - tell him you have her number and you may well talk to her - but make sure he from now on tells you of any contact and please make sure you are prepared to stick by any ultermatiums you make.
Seek legal advice too as you will feel so much more in control too.
Good luck - it can work out but make sure you now look after yourself.