This is SO exciting to read!
I have always though of processing differences as a defective radio. DS has been diagnosed with CAPD after years of thinking his "hearing issue" was conductive. We are inclined to think that Bee has the same issue, but her inability to follow even simple directions makes it hard to even test for it in the first place.
When I was in the Coast Guard years ago, if there were technical issues on the radio, we used "words twice protocol". For example, if I wanted to say "what is your bearing, over" what I would actually transmit was "what what is is your your bearing bearing over over" so that if it broke up, the receiver could piece together what they had heard and make sense of it.
I imagine that, in many cases, either the sound is going from ear to processing so slowly that the brain is trying to assimilate the first sound of the word just as the last one is being heard, so the last sound being heard gets "forgotten" or the brain is just so overwhelmed at trying to make sense of the beginning it omits the end. By speaking words slowly, you are allowing DS3 to hear and assimilate the beginning of the word before the end of it "disappears" prior to making it high enough in the queue to be comprehended.
As for mature sentence structure but immature sound, Bee (and to a lesser degree, DS) cannot follow instructions. At all. Not even if you pay her. Interestingly enough, not even if you demonstrate... DS is "in one ear and out the other" as well, but I am not sold on how much is CAPD and how much is 14 yo boy with his own agenda.
I was told by a SALT years ago that this disparity between expressive and receptive language (where expressive is ahead by 2 or more standard deviations, and receptive is significantly delayed) was unique to children with shunts or hydrocephalus - she wanted to know if Bee was shunted. No - she is just doing her own thing, I guess.
The SALT described Bee's speech at the time as if she was reading from a German phrase book. She knew what to say and how/when it was appropriate to say it, but if you replied to her in the language she was speaking in, she stared at you with a blank look as if she had NO idea what you were saying.
Through inner ear testing, we know she hears well, but since behavioural testing is beyond her ability at the moment, the rest is guess work. DS, otoh, apparently DOES have a hearing loss (this has been something that has been open to debate for years now - yes, no, maybe, it is behavioural, it is processing, it is conductive... ) but his processing issues are unrelated to it.
As far as DS3, though, now that you have found something that works for him, run with it as long as you can. Perhaps you have just opened a door - a very exciting door with tons of potential...