A cheap way to realize low pressure differences (and a method that is nearly unbeatable for smoothness) is gravity. Fill everything with liquid, run input and outlet tubes to two beakers/buckets/etc, with the one on the input side be higher than the output. Approximately 1 mbar per centimeter height difference for water at room temp.
This is a challenging measurement. In my opinion best done with a very good balance. Fully shielded from thermals and vibration, obviously. Use a small container with small opening diameter to limit evap to what will hopefully be under 10% of the final measurement (and we will correct for it more later). A clean 2mL glass bottle is good. Fill bottle halfway, ...
Same answer as above, "Yes BUT..."
This is from a practical point of view:
Other than HPLC, nearly all "microfluidics" applications are far into the laminar region. Poiseuille's law works very well most of the time.
When going below 100$\mu$m dimensions, the problems come.
lack of stiffness -> violates assumption that fluid into a ...