You missed the "ecommerce" in the OP's first line. That will require an SSL and that will require a dedicated IP address and use of the (old) domain name. Further, I believe that WP requires configuration with these same details (perhaps I'm confusing this with ZenCart?). Therefore, IMHO, the new website needed to be developed OFFLINE and should have used a local domain without the TLD of the online domain (easiest to "backfill" when moving online).
I thought that the OP's reuse of WP meant that the old pages remained ... which implies to me that no redirects should be required.
With the limited information available, it doesn't appear that mod_rewrite can help with the structural problems which are implied by the questions.
... my web developer (in whom I don't have a lot of confidence on this issue)
You appear to have used a non-professional "web developer," one who neither knows what he/she is doing and who doesn't care (vacation is more important than doing the job correctly). This may be a "you get what you pay for" situation but you're stuck with a non-professional website and are also suffering the loss of your Google PR - how much is that worth?
Steve gave some good ideas about finding someone to do the job correctly but you need to get someone REALLY good to try to undo the damage before it becomes irreparable. Don't waste time, get it corrected ASAP!
Again, I don't know why you guys are so good about answering these questions, but I sure do appreciate it! Thanks.
For me, there are large portions of "been there, done that" and "professionalism" (something your developer obviously lacks). I've seen too many horrid websites and could only shake my head. :nono: I have spent many, many hours here trying to help spread knowledge so members don't have to rely on the "cowboys" simply to repay the few who have helped clarify some things for me along the way. My reward has been acknowledgement by members who have sent thanks via posts and PMs to express their appreciation.