Was it a mistake to book a BA flight through AA?

TLDR: Paid for free seat selection through AA, still being charged $$$ by British Airways to select seats.

I use google flights to plan out most of trips, and as is often the case partner flights are displayed with a US carrier as the primary.

Booked on American, paid a extra for "Main" to get points, seat selection, travel credit if needed, etc. I knew it would be a BA flight but figured as partners all of this would be honored.

Buy tickets, get directed to BA site to select seats. To select a seat, in ECONOMY, it ranges from $175 to $89, per seat! So to not be at the last seat on the plane, I'm looking at around $120 per seat, so $240 round trip. Thats like a quarter of my ticket cost.

Called AA and their best recommendation was to cancel my "Main" ticket and buy the Basic (stripped of all rights) ticket and then still pay the $240 for the seats, to help offset the cost.

The other recommendation was to just take whatever seat is assigned at the gate on the day of flight. Thats saves me the money but also means I may get a middle seat in the back row for an 8 hr flight.

Called BA, and in such a polite British way, I was told thats not their problem, and to just take the free assigned seat.

My questions:

1. Has anyone run into this and have you found a way around it? Meaning, can I get BA to honor the free seat selection OR get some money or credit from AA?

2. What other benefits of a Main AA ticket did I forfeit because its a partner flight?

For context, I belong to the loyalty programs for both airlines but sadly not enough status to mean anything to the airlines.