Jon, I think the answer to both of your questions are linked.
If you are try to bond PBT (and a lot of other thermoplastics like ABS and Polycarbonate) with a cyanoacrylate or a solvent based adhesive you want as much surface contact as possible without air gaps.
If you are using an epoxy it doesn't bond to the plastics very well and a lot of the success of the bond is down to what texture there is for the epoxy to form around before it sets giving it a mechanical grip on the parts.
Also remember you can join thermoplastics quite well with a soldering iron - [You must be registered and logged in to see this link.]
I have successfully repaired the indicators on my wifes Breva with a $2 soldering iron and used cable ties as the welding rod