The problem of just looking at Aeroplan site is, you have already found out, what is shown on Aeroplan site, is NOT what shown in partner's system. So you are being misled into Aeroplan site to think your itinerary is all good and dandy, until you call to get seat assignment. It is good to know Aeroplan eventually take good care of you. What if they could not?

You could have avoid all the last minute "excitement" by checking partners' sites one by one and then probably found out the SQ or even the OZ and BR bookings were not there since your last Y -> J change,

As you can clearly see from my posts, AC site gives a completely screwed up display on our itinerary. If I dont know any better I would freak out seeing the inbound from Hong Kong on BR was completely GONE, not to mention UA segment was stuck between the 2 TP segments on the outbound...

Just HOW I can look at the Aeroplan site to know if the itinerary is properly booked, EVEN IF I prefer to see it on Aeroplan site too - because that is their darn miles I used and it is 014 ticket stock! I want to make sure everything is displayed as it should be. Unfortunately the reality is, I cannot trust the itinerary display on Aeroplan site as it is a complete mess. I cannot tell if BR flights still exist for example.

Cannot agree more. I find the reservations are displayed properly on each partner's site for their own segments plus some (definitely includes the incoming segment before their own). The big thing is, BR shows all the segments on their flights are INTACT while the whole thing is NOT found on Aeroplan site.

Partmers' PNRs are immediately available as soon as the reservation is made, does not even need to wait for ticketing. I know this from AA which allows a 5 days hold, and before UA / DL changed their hold policy to immediate ticketing - once the agent of the program you use, finishes the reservation, all partners involved in the reservation would generate their own PNRs. Just ask the agent to give them to you while you are still on the phone. I could not do that on this booking because the phone went disconnected, presumably due to the Aeropllan system went crashing down.

I am super surprised though by how UA site shows the Full itinerary AND even produced a Receipt with FULL breakdown on the taxes despite UA is not the ticket issued airline and its role is only the Last Segment to get home (in Y unfortunately). In other words even UA lose our booking it is no big deal, but UA actually shows the full picture.

On top of that, partner sites often show schedule changes, even flight canceled and rebooked, as they happen - but the award booking often has time lag before it displays something changed, or a big message to tell you to call (this is how AA, AS and UA handled their award bookings). When I saw those messages on the award itinerary site (the airline which program used to book the ticket), the first thing I do is to hop on partners sites to check what changes are - sometimes just a normal schedule changes, sometimes can be a misconnect, sometimes a flight cancellation and rebooking ... all sorts of things, then I can decide whether I let the changes sit (normal schedule changes that I suspect would have even more such due to seasonal adjustment) or somethings needs more urgent attention such as misconnect - I can figure some alternatives / solutions, before I call the award program and then feed the agent MY Preferred Solutions, insktead of finding out what it was and then what the remedies might be, at the time of the call.

I dont know, I am of the belief that I would be the one who is most interested in protect my trips - i.e. I would be much more proactive to make sure things are done right, and when they go wrong, I learn about it much sooner therefore have much better chances to fix it to my liking - just the hard-learned lessons from years of booking award travels, often involving 3 to 4 programs each year on 2 to 3 month long trips plus short trips. I think the highest number of programs used in a trip happened in 2016 or 2017, used AA, AS, UA, and BA all on one single trip. 3 programs used on a trip is a norm. The Aeroplan booking is an exception and I suspect a one-time only thing as we all are quite pessimistic on how AC would do with the program. LOL.