It's very difficult for me to reconcile the idea of a dish that aims in a particular direction with the idea that deflector beams can be directed in any direction with equal ease.
It would help if we knew what a deflector beam actually does. But, just like the Bussard collector, this technology has one definition in noncanon, backstage books but none in the actual episodes or movies. All we ever see or hear it do is "alternate modes" or "special adaptations", with nary a hint to what its normal mode of operation would be.
I don't think that's true. In the wormhole segment of TMP, Ilia tries to use navigational deflectors to keep the asteroid from hitting the ship, but they're inoperative. That would seem to be their normal function, established canonically.
I'm going to call bullshit on the Enterprise
being unable to blast out of orbit on Delta Vega if their attempt to regenerate the warp drive fails. If they can make it to bases that are yeeeeaaarrsss away, they can blast out of orbit of one freakin' planet and escape its star system. If Delta Vega would trap them like that, then it's going to be the skeleton of Kirk's great-great-great-...-great-granddaughter who makes it to the nearest starbase.