The two examples in the OP are not due to any fault of the Canadian Governments IT department, they are user error. Why say you're a subject when you're a citizen, only very old passports or possibly still some valid overseas territory passports state "British Subject" in the citizenship area. Selecting that when you're a citizen is an error in my book.
The EU passport holder who only put 2 of their passport digits in...why would that be accepted? How is that the Canadians fault?
If your applications says it's Provisionally Accepted then you would surely have the sense to check again closer to travel time and if it's still only provisional then you'd maybe get in touch with someone at ETA to see what's going on? I'd never read "Provisional" and think "I'm ok to go then".
The suggestion to fly via SEA/BOS is also going to require an ESTA, which is just as likely to give a traveler into bother as I lost count of the times when I was dealing with USA bound passengers, that they had filled out an ESTA incorrectly, they ignored the fact it had been denied, didn't realise they needed one or a perfectly good ESTA application was nowhere to be found on the check in system, so a lengthy call to get a manual override was on the cards. This is still something I hear of today, nearly 10 years on from ESTA introduction.
Exactly the same story with the Aussie ETA as well.