Very correct, even after commissioning of the Hassan-Shravanbelgola line, the bottle neck of Subhramnya Rd ghat section will be there. Only benefit derived from the like could be reduction in journey time between SBC & MAQ by 1-2 hrs.
Short time solution could be running all the passenger trains with 26 coaches.
Medium term solution of making necessary arrangements to overcome any shortcomings coming in way of using crossing stations in the 55km ghat section, may be relatively easy to implement
But...
more... the Long term solution i.e. doubling, as you have said looks very difficult.