Trondheim is...
Several times during the day sas.no and norwegian.no
The best way to get from the airport to the city centrum is by the airport busses, or by train. Taxi is also an option, but rather expensive.
Bus: https://www.vaernesekspressen.no/en
Train: https://www.vy.no/en
Taxi: Taxis outside the airport
More information about ground travel: https://avinor.no/en/airport/trondheim-airport/to-and-from-the-airport/bus-train-and-taxi/train
Trondheim is a small and compact city, so several hotels are within walking distance of the conference hotel. Here are some examples in different price categories:
Britannia: From about 300 Euro per night
https://britannia.no/en
Clarion Trondheim: About 200 Euro pr night https://www.nordicchoicehotels.no/hotell/norge/trondheim/clarion-hotel-trondheim/
Thon Hotels: About 160 Euro per night
https://www.thonhotels.com
P-hotels: From about 80 Euro pr night