I think the simplest explanation is the likeliest.
She needed urgent but not emergency surgery for something like Crohn's, or a scheduled minor op turned into something much more serious. A big op with a long recovery - even without a stoma, having some bowel removed is a big deal and takes ages to get over. For example, other surgeries are available.
They were all a bit blindsided by it and didn't have time to come up with a PR plan. Keeping her medical details private is extremely important to her (makes sense given she has only shared the bare minimum in the past), which is of course perfectly reasonable and very sensible.
To protect her privacy they've kept the details to a very limited close circle within the families and the KP machine, so most people working for them don't know what's up and they're running the PR on a skeleton team by choice.
This leads them to fuck up, a little bit with the "personal matter" at the memorial, and then royally (
) with the photo. The photo happened because they didn't have the correct people advising them, because the correct people don't know what's what.
She still wants to keep everything private and doesn't want to show her face until it returns to normal - which is sad but understandable on a very human level.
The press are now fuming that they have no idea what's what, and that they were lied to with the photo (which WAS a big deal, even if it wasn't hiding anything more sinister than moon face), hence the Rose articles.
There was no ambulance. There is no evidence it was her or anyone royal in that convoy. The two pap shots are legit, but the one with the face turned was planned and authorised.