I think you are right as to the solution but the whole approach was...
....umm, backward.

What is normally done here is it feed the yeast slowly but start the
must at a normal PA. There is just too much sugar here to start
easily.

As to more yeast; absolutely, but all of the same kind and you already
picked a winner. I might make a starter from 3 packets of yeast if I
were you and warm the mass of must up too. Beer makers do this all the
time, it's cheap insurance.

If it's possible to pull some off, maybe dilute it to a PA of 12 or 13
and get that going really well and then when it gets to say, 1.030,
start adding back the rest of the must.

How much must do you have now?

Joe