I need to fly DUB-SFO return in the next month. The fares for this route seem to be about $900-1,200 on AC as well as other airlines. I expect AC to involve a connection at LHR and YYZ or other Canadian city.

As long as I'm stopping at a Canadian city I want to work in a stopover in Montreal. But AC's multi-city planner seems to get confused by the DUB-LHR leg and only generate fares based on separate fares for each leg. As as result the price skyrockets to $2,000+.

If I just input the flight as LHR-SFO with a stopover at YUL then the prices are somewhat more reasonable but even then I'm having trouble. The multi-city planner still generates separate fares for LHR-YMQ and YMQ-SFO. I suspect what's happening is that there is limited availability on some legs so separate fares are cheaper than the more expensive fare across the whole trip. But the multi-city planner doesn't let me see the price for all the options so I can't figure out which option has reasonable prices.

Is there a way to see what flights have availability in reasonable fare buckets and then force the multi-city tool to generate the right combination of fares? I would expect a stopover to add $100 plus tax to the base price, which is what the fare rule says, so it ought to come out to about $1,100-$1,300.