I agree with pp that LGW is fine-Heathrow is out of the city too-just in a different direction.
I don't think, unless you pull a 125K personal plus a 100K biz that it would be realistic to have enough MRs to book next summer. I think you should stay the course BUT start diversifying and adding to your MR stash so when you need them you'll have them.
Again as
@GZanta77 said, it's about 5 hours ish. The nice part is Waverly station is right in the middle of Edinburgh and easy to catch a cab, or if you stay at the fancy Balmoral, walk to the hotel. Trains are nice because you can get up and use the restroom anytime and you can generally move around more freely. You also won't have to worry about pressure and baby's ears on the train (v flying which may or may not be an issue). Every single mile of the ride will be gorgeous once you hit Scotland, too (I kid, but as you know I'm biased
).