Mini Split HVAC System Size for a 1700 Sq Ft basement,,,

I hope the 90K is the output or it's truly undersized! Even so, it's quite common to oversize furnaces beyond the calculated heat loss by 20-30%. It's also possible the heat loss calculation is low, it's really just an estimate, and part of why the furnaces are oversized. Even if it is undersized, it would not significantly alter your energy costs. Costs are dependent mainly on the home's overall insulation and the furnace efficiency, the capacity and running time is not a big factor.

Changing ductwork will better distribute the heat, but if more heat is leaking out than the furnace can generate, it will not help. While a larger furnace will better heat your home, it will not help your energy costs. If your costs are out of line for your type and size of house in your area, then more heat is being lost than is typical. In the long run, you are better off preventing heat loss than generating more heat.

There are energy consultants in most areas that have the proper tools and equipment to advise you where most of the heat is being lost and what could be done to most effectively reduce heat loss.

Try closing all of the other vents, this should blast that room. If not, then that room's duct is either grossly improper, has a blockage or the ducting has separated. If it does blast that room, & hopefully it does, then you'll want to keep that configuration for the most part. It's called Balancing & you'll want to just crack the other rooms & hall to a little bit more open for an equal or Balanced distribution or you may find that just 1 vent (hall) really affects the Balancing. You'll have to remove the grills that don't close & block them with cardboard inside the fully re-attached grill for testing or Balancing & also to repair their operation or replace them.