Effectively owned in a legal sense would be having pretty much the same bundle of rights that an owner has. Thus a rental car is NOT effectively owned by the renter. He has possession of it, but does not have all he same rights that the owner of the vehicle has. You can have some other definition of the term, but that doesn't help the OP with a legal problem.
As I said before, the USPS has rules for the placement and design of the mailboxes, analogous to building codes for a home. There are also statutes like the one you cited that penalize the damaging of mailboxes and the theft or destruction of the mail, much like state and local laws against vandalism, destruction, and theft of property. But it does matter who owns them because the USPS does not have complete control over a mailbox you own as it does over the ones it owns. Though there are clearly rules that apply to mailboxes the laws and USPS rules as they apply to private mailboxes are not as extensive as you and Harold make them out to be.
In any event, it obvious that the mailbox set up itself meets with USPS approval as it is delivering the mail to the house the OP lives in. The only change here is that the homeowner is taking the mail once delivered and distributing to the OP rather than the OP getting the mail out of the box himself. There is no federal crime committed by the homeowner in doing that. The homeowner is not stealing the mail nor, in the words of the statute you cited, does the homeowner maliciously injure, deface, or destroy the mail deposited in the mailbox. Nor is what the homeowner is doing affecting or changing the way the carrier delivers the mail to the place. From the USPS perspective nothing has changed for the postal service and the OP is still getting his mail. Given that, while the OP is certainly free to complain about the homeowner to the USPS about this, I suspect that the USPS really won't care. Still, a complaint to the local postmaster is free and there is an off chance the postmaster might talk to the homeowner about it and get things to change.
If the USPS doesn't care then whether the OP might have some legal remedy depends on the answers to the questions I asked earlier, which so far he/she has not come back to answer. Even if there is some legal remedy, the cost and time to pursue it might not be appealing to the OP. In the end it is likely to come down to how well the OP can negotiate some acceptable arrangement with the homeowner that the two of them can live with.