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As dunk said, the schedule change is as a result of the political situation in BAH. I think you need to have lower expectations of BA's obligations - it's not as if they changed the flight on a whim. Your onward connection doesn't help things, and this is one of the well-documented perils of separate bookings.

Have a look at the viability of cancelling either or both of your itineraries, and rerouting, if possible.

'Not wanting to spoil the stopover and having nothing to look forward to on the return' sounds like a fairly weak argument toe, and I would leave it out of the equation if I were you.

It might be nice for them to comp you the seat selection, but as this is a (semi)-voluntary downgrade, you might bit get far.

What does your travel insurance offer?