Hard to say without seeing it. Keep in mind they try to access everything from the driver's side of the vehicle without getting out. The post office also can require things be clustered, made easier to access, etc.
If you super duper think you have a better solution, you can try to debate it with the postmaster. However, they're almost certain to rely on the delivery person and their request for everything to be clustered together. Delivering mail to everyone can be a big physical burden, and they need to make it as efficient as possible.
Yes, if you don't comply they're right to hold your mail for pickup and return it if you don't. If you really want to dispute everything, get ready for them to require you to come pickup your mail throughout the process. Consider them delivering it to you (as opposed to forcing you to pick it up) to be a privilege, not a right.
No matter what, your solution appears to still be requiring two separate stops at the minimum. I'd bet there's a problem with how she has to access things as well. If it was me, I'd just move the darn mailbox. Don't get the postal people unneccessarily miffed at you.