25mg of sinemet is a tiny amount of drug, i doubt any gets to your brain. it does seem to be affecting in an extreme way the cells that affect your balance that are outside the brain.
is there a remote chance you are sensitive to the yellow coloring? can you tolerate a baby aspirin which is usually yellow?
the 25/250 tablet is blue, you could dissolve it in water to get a small dose.

you can dissolve a 25mg of a 25/100 tab in 2 cups of water and 200-500mg vitamin C powder and drink half of the mixture to get the l-dopa down to 12.5mg. you can use whatever proportions you want with this system.

it would be interesting if you could do an experiment and try a placebo to see how much of this is mental, would take a friend to prepare the liquid l-dopa and the placebo.