They did speculate it, and maybe she wasn't sure she'd survive the summer. But clearly she didn't want to change her summer plans, wanted to visit Balmoral as normal and if she happened to die while up there, then so be it. She is, after all, half Scottish, and all accounts say it was she felt most normal. Thinking about it, it wouldn't have been too much longer before her return to Windsor would it? Mid-September? So just in time from that point of view. I think her send off has been lovely so far.
As for the Unionist argument, I'm sure some will make that case, but Nicola Sturgeon is a huge admirer of the Queen and her blueprint for independence intends for Scotland to become another Commonwealth Realm with the King as Head of State in an independent Scotland. In other words, they can have independence and keep the monarchy is their aim.