my man's mum and dad had a bad divorce 6 years before we got engaged.
We never had an engagement party but my chief brides maid did organise a get to know you party on my home turf but his father didn't show. however when my family met his at the wedding they got well. i'd suggest you get one of the wedding party (who can act stupid to family politics) to orginis an informal get to get in a neutral venue.
As for the engagement party...why make folk buy two presents!!!! if there IS to be a wedding the engagement is only the announcement for the wedding!?!?!?!? is it not?