I recently had a (in tank) fuel pump failure just after a fill up. I tried to siphon the fuel in the time-honored way by sticking a tube into the filler and sucking out into a container. It didn’t work and I ended up siphoning directly out of the tank from within the car. This requires a lot of dissambly.
Give it a go with the old fuel. What’s the worse than can happen? Nothing damaging would be my guess…
Obviously don’t run it dry to get rid of old fuel, just dilute it as you have suggested.