external 3.5mm dongles worked for early prosound mods.

(something like this)

It's up to you. I'd personally just go and try to do the internal jack seeing that fixing the external ones isn't working too well.

that really would be the best solution, albeit a scary one (mainly because of the extreme lack of internal space in an SP).  i guess the question also would be whether i'd want to go all the way and try and do an actual prosound of the SP (the ideal goal) or to only do just enough for an integrated headphone-in.  based on my research, each method comes with its various pros and cons.

at the moment, i'm just annoyed that i may have to order yet another headphone adapter...

Maybe you should try this:
http://www.youtube.com/watch?v=lWBAqTu7NKY
(It's a video of a guy who added an internal headphone jack)

Anybody ever have any luck trying to get one of these ...

.. fixed by shortening or removing the cable altogether (in other words, the headphone 3.5mm jack is basically connected directly to the base of the GBA connector)?

i've already gone through 3 headphone adapters, and i'm thinking of toying around with at least one of them to see if i can revive them.  that said, i'd be gladly willing to entrust the repair of these to someone who knows what they're doing.