if refusing place at 13+ London School, for older DS, will it affect younger sibling in the future?

I would say yes.... I'm fairly sure DS2 failed to get 2 separate school places because they knew DS1 was at a different school.... I think schools do look at older siblings when deciding whether to offer to younger ones.

No. Schools are used to people moving house and changing their minds.

I'd say maybe too. Depends how oversubscribed the school is? We didn't get an offer this year for very bright DS, because we had turned the school down previously.

They won't like being mucked around.

Day/boarding choices are very different especially age 10/11. I'd be honest, let them know that you are accepting the place but are waiting to find out results of a boarding school and are still very undecided between day/boarding. They'll be glad of the info as it helps them manage their waitlists and keeps them in the loop and in contact.

Hmm, I would say maybe. You couldn't blame the day school for thinking there's a pretty good chance you'd do the same again with your second child?

no