He can do it. I believe you mentioned he could. The problem is not all fares will price the same when combined onto one-ticket.

Note I made the comment on SHA because in one of your posts you say you're going YYZ-DTW-SHA not DTW-PEK-SHA. Finally having a non-DL segment may or may not help/hurt you iF you need rebooking.

Westjet operates from terminal D at LGA. I presume Delta does the ground handeling as well.

It has taken 2 pages of posts to get simple & accurate information which changes the entire picture. It would be a good thing if OP checked his tickets for the operating carriers and times because none of this will matter if he doesn't get them straight.

It doesn't matter what the "DL lady" told OP on the phone because OP is on a WS-operated flight and if WS causes a delay into YYZ, it is WS which is responsible for OP's onward travel if there is any on his ticket. But, his ticket terminates at YYZ so far as WS is concerned. Thus, WS will politely smile at OP and tell him that it has met its obligations. End of story.

OP then appears to be booked onward on a DL-operated flight which he will have noshowed for. At that point, the DL ticket will have been cancelled. This will include all onward segments. Will DL reinstate it? Perhaps. It will take time and it will take effort and because there are other onward carriers involved, it is not as simple as DL waving a wand (and waiving fees pardon the pun).

Thus, the key is to allow oodles of time to allow for IRROPS, border issues and anything else that can go wrong. A cheap airport hotel would make sense, lower stress and serve as a smart insurance policy.

I just did a similar trip -- DCA-JFK-YYZ (all DL on an award ticket) on one ticket and then YYZ-JFK-AMS-KIX on a KL ticket, DL operated YYZ-JFK.
I did risk a same day connection but with a much longer connection time. I gave myself about 4 hours in YYZ just in case there were problems. It worked, but Canadian immigration took about 45 minutes. Security and immigration back to the US was pretty quick, but I do have GE.
I did a same day return connection as well. Allowed 4 hours for the connection in YYZ.

I'm a risk taker when it comes to combining tickets.

I've been accommodated (very well!) by delta when a two-ticket itinerary created a problem.

I would absolutely position the night before in this case, if it was me. I do not like to cut things close on the outbound. In addition to the potential costs (which could be zero), also consider the disruption to your enjoyment of the trip if you get a misconnect on the outbound.

Misconnecting on the return is generally less serious because in the worst case you must pay to rebook or purchase a new ticket for the last "domestic" positioning flight versus needing to get a new longhaul international business class ticket. Also, there are generally more flights so you're less likely to be delayed by a day or more if there's a problem.

This post reminds me nearly exactly of the recent post where a couple flew a discount airline from Bilbao to BCN??. The flight was late, and he missed the DL award flight in J to the USA, and DL wouldn't do anything, and ended up paying $5600?? to buy new J tickets - and blamed DL for not helping him.

IMO, if you have checked bags - FUGGEDABOUDIT! Without checked bags, you hava shot a making it.

Again, as mentioned above: You'll hafto clear Canadian immigration and customs. Deal with bags, if any. Get to DL - check bags if any. Clear US immigration and customs, then security. Possible long walk to a gate.

When I flew the NW Bucharest deal out of YYZ, I think I allowed most of the day (6 hours?) between my award ticket and my YYZ/AMS/OTP legs. I DO REMEMBER that, when I told the Canadian immigration officer that I was going onto Europe, they sent me to a secondary immigration screening area for more thorough questioning!!!

if you got these two tickets from delta, have them link the two reservations. i do this quite often having to buy a separate ticket to connect on a long haul.

I personally don't care if I miss my flight (as long as I get accommodated) - hell I'd probably get a more desirable routing - maybe even 747 through Narita - I am just concerned about losing $$$.

But since they are both through Delta I'm going to gamble.

Linking does NOTHING. It simply results in a note in the reservations system. No aiirport agent unless they go looking will ever see any note about the other reservation. Even if it's on one reservation, if it's on two different tickets then airline flying you on ticket one has no responsibility for flying you anywhere on ticket two.