Would a ducted heat pump lose a lot of effeciency if the ducts are installed in an open crawlspace?
If flex ducting is used it comes pre insulated.
If you decide to install a split system make sure it can handle that level of cold.
Yes, some can but they are high end units.
If you need “emergency heat” or supplemental electric it gets much more expensive to run.
I find when you look at the added cost of installing ducting a “mini split” is a better choice. Mini splits have the outside unit but then run lines inside the home.
Each room or zone has its own heat /cooling unit.
Each room or zone can be controlled separately. The cost of ducting can pay for several separate inside units and you wont have the air cooling in the ducts even if insulated.
Mini’s can be cheaper to operate because they are not heating dead air space and if 1 room or zone is not needed to be heated or cooled turn it off and the outside unit slows down drawing less power or cycling for older technology.
What ever you decide make sure your unit can operate to the -7 range, I believe the best one I put in can function to -5 but some can go to -15. They are not as efficient at these extremes in any split system but depending on the brand / model they can do the job. they go into defrost mode more often.