looked at video #2 and here are my comments
1. didn't think l-dopa nor carbidopa was absorbed via the mouth, takes active transport in small intestine? trivial point. the liquid sinemet procedure from what i understand is more to accurately divide the tablet sinemet into smaller doses to be taken more frequently, the sinemet pill dissolves awfully quickly imho.

2. i think you need to establish if the l-dopa is actually getting into your brain, and once there is it getting converted to dopamine, i.e., do you have a gastric emptying problem which extends the time the sinemet stays in your stomach and as a result a lot of it gets converted to dopamine which doesn't pass the BBB? there are tests for gastric emptying, not sure how easy or reliable they are. and then once in the brain, do you mfg enough enzymes still to convert the l-dopa to dopamine?

3. the apomorphine i assume will get into your brain quickly since it is injected. if it helps, it will show you still have functional neurons that have dopamine receptors, which might imply the l-dopa either isn't getting into your brain and/or isn't getting converted to dopamine.

so maybe apomorphine might be used just as a quick diagnostic? if it helps, maybe you can decrease the sinemet and add mirapex or requip which doesn't get broken down in the peripheral tissues rapidly and don't have to be converted in the brain? they take a while to titrate up. again, not an expert you need to talk to neuro.

4. peripheral neuropathy not a clue, sorry.

finally, carbidopa doesn't pass thru the BBB, if it did, the l-dopa wouldn't convert to dopamine, i have read speculation that at high doses, greater than 200mg of carbidopa, it might pass thru the BBB. just wild speculation here, might not be a shred of truth there.