Why are you quoting YEG-ICT fare pricing? Do you really think you could just combine a YQU-MSP fare with a YEG-ICT fare (and just skip the YEG-MSP segment)?? The fares you are quoting are O&D based, not hop-by-hop. Do you mean the fare on MSP-ICT DL5041 which goes for USD $324 in Main on March 4th. This is a DL T fare and it only allows fare combinations with other DL fares, not WS fares. These restrictions are found the "Combinability" section of the fare rules which are available on ITA and delta.com. In order to combine the DL MSP-ICT fare with a WS fare on a single ticket, you need to upfare to a DL K fare which is quite a bit more than the T fare. The combinability rules on fares can't just be overriden by an agent. ITA is showing the DL K fare on MSP-ICT segment when you price this all together. So, yes, the itin seems like it should be bookable for CA $829.98. But not the price you apparently think should be available.

DL site almost never shows partner fares with searches so it shouldn't be surprising it is not showing given it involves a WS fare. It's also a broken fare and DL site will rarely show these as it mostly only shows O&D fares on a search. DL has YQU-ICT fare filings, but they don't look to have a codeshare on WS3390 which is why it won't price on a single DL O&D fare. Not sure why Priceline is not showing it, but Google Flights is not showing these fare combinations when I try pricing it there using multi-city so there may be something else going on here. At any rate, it's going to be cheaper to purchase these as separate fares on separate tickets rather than trying to combine the fares on a single ticket due to fare combination restrictions on cheaper fares.