Pardon my ignorance - is there still a "male line of succession" rule that takes precedence? I thought that was settled back in Elizabethan times.
Yes, blame us.

Canada has a lot of great things going for it, but the Sussexes may just find it's a nice place to visit, but you wouldn't want to live here. And the weather is the least of it.
On the one hand, Canada is very, very multi-cultural; heck, by census data there are more non-white people in Vancouver than there are white people, if that's an issue for anybody. Racism? To a point I suppose, but nothing near the tensions that apparently exist in the US and parts of Europe. A mixed-race couple and their children wouldn't cause an eyelash to flutter. We also have nothing - really nothing - that resembles a tabloid-type press and no paparazzi. We are not obsessed with what celebrities, politicians and the financial elite are doing.
On the other hand, that could also be an issue. In a relatively egalitarian culture with no entrenched history of a class-system, it might be hard for them to get the adoration they may feel is their due. As much as they say they want to live normal lives, I don't quite imagine anonymous Joe Six-Pack and Suburban Soccer Mom is what they have in mind.

In this case, if they intend to reside in Canada, or even spend any substantial time here at all, the burden is on us, as taxpayers. Every member of the British Royal Family (and countless other people with diplomatic status) are designated as "Internationally Protected Persons" in keeping with the UN Anti-Terrorism Treaty. What this means in the case of the Royals is that as signators to the Treaty, Canada is 100% responsible for their security when they are on our soil. The RCMP (our national police force) would bear the burden for whatever is deemed necessary. They may supplement with private security resources at their own discretion but that still doesn't get us off the hook.

Very good point. Apparently the relationship between Diana and Sarah devolved that that, for whatever reasons.

Depends who you ask, I guess. In general Canadians have a sentimentality about the British Royal Family; we are, after all, the Queen's Loyal Subjects. As I mentioned earlier in this post though, we're also extremely egalitarian - the very idea of elitism doesn't sit well. And economically speaking, there are about a million things I'd rather see my tax dollars spent on than their perpetual security costs.
Yep - regency, which has been used down through history for situations like that, amongst others. Interestingly, there has been suggestions of a Regency now in the case of Queen Elizabeth - who on the other end of the spectrum may feel "too old" to do the job properly. I've read several articles speculating that rather than abdicate to Charles, he may be appointed as Regent while she retains the Crown until her death. Not a bad plan, really.
Since Regent is an appointment, if it were necessary it would not have to be the next person in line. If a disaster scenario befell the Waleses, I'd prefer to see Ann or Edward take the helm. They have both lived their adult lives with the dignity and decorum befitting the station.