I don't know what motip hammerfinish is.

I just recovered mine using vinyl cloth. Although it came out pretty nice, it certainly isn't factory perfect.

I used the spray foam brand name Great Stuff for filling the cracks, but if I did it over I would probably try silicone.

The skim coat of fiberglass putty would probably do well towards stabilizing the repairs. Long fiber is what I would try.

You can also ship your dash off to receive a factory perfect reconditioning.

As for adding a speaker: if you are planning a non-original finish then consider using a grill, as it would be easier.