I would like to fly LH and AC on DOH-FRA-YYZ-HAV. If I try booking a RT on this route with LH I have no issues.. No time conflicts at all.

Returning from HAV, I can book Swiss Airlines DOH-ZUR-HAV and then HAV-ZUR-DOH, RT with no problem at all.

The difficulty occurs when I want to combine flying to HAV with AC/LH and the shorter route with Swiss Air, as in DOH-FRA-YYZ-HAV-ZUR-DOH as combined route.

Search engines generally do not allow this search because of the US-based blockade. Exception is Zuji, but it gives limited results. Yes, paying for a telephone-assisted reservation may be the only option at this stage. Chickenfeed as compared to solving a problem with getting routings/flights I want.