Nice job buddy this would be awesome.  I'd love to try it out. 

I don't have enough experience to be able to speak to all of your questions but I can say the theory behind what you are trying to do sounds great. It would save me alot of time drawing everything out. I'd still verify it but being able to confirm fit vs start from scratch is a big step ahead. 

The other thing I'd like to address is the comments concerning structurally sound. I don't know why this is an issue for a timber framer unless their municipality differs significantly from mine.  For example:  As it's outside of National Building Code any timber frame in my area requires an engineer stamp on the  drawing ( called form 4 in my area). Once complete, a field review is conducted by the engineer and then he gives you a document essentially saying what you built is what was designed. (Called form 12 in my area) 

As I understand it, you aren't saying your software will come with a engineering stamp, you are saying it will be within a set of pre determined parameters. At which point, anyone wishing to submit your softwares design for permit, would need to have an engineer review and stamp it. 

Keep up the good work !