I disagree strongly (and I'm a Canadian citizen!)
#2 and 4 are really questions of public opinion. A poll in the
National Post showed that
60% of Canadians would approve of Prince Harry becoming Governor-General, including 62% of 18-34 year olds and even 47% of Quebecers. By contrast, an Angus Reid poll showed that
55% of Canadians approved of the incumbent Julie Payette's appointment in 2017.
As for #1, I don't think anyone is suggesting booting Payette immediately. I imagine he would be appointed at the end of her term in 2022-23. This would give the Sussexes 2-3 years to bolster his finances, place down roots, and enhance their reputation further and take on local patronages (Colonel-in-Chief of the Royal Canadian Regiment?).
#3 is the only real barrier, but it's hardly an insurmountable one, if the Prince is willing to tackle it . In any case, the informal convention of alternating between Anglophones and Francophones means that the next GG should be an Anglophone, so I imagine he has some leeway (and bilingualism too is not a a hard rule but a custom which is flexible to public opinion, although given that it's expected generally of high-ranking officeholders I imagine it is less flexible than #2 and #4).
I think this would be the best of both worlds. It would lend the office of
de facto head of state a glamour and prominence on the world stage it's long lacked. It would allow the Sussexes to take on a role with the purpose they've lacked in the UK, but one they would-- given their unique stature-- they would probably have a lot of room to redefine. It would abrogate all sorts of financial concerns. It would revitalize the Canadian monarchy for and renew the Crown's link with the largest Commonwealth realm during what will probably be an uncertain transition period. This Canadian move may yet prove a very sound idea, even if the roll-out was botched.