1 milliliter = 1000 microliters

You can't directly convert milliliters to milligrams (volume to mass)
without knowing more information such as the specific gravity.

A 30uL bottle? 30uL is only ~1-2 drops.

You need to clarify the question. Are you trying to dilute a liquid of a
specific concentration? If so, what concentration? You can't dilute any
liquid in a 30uL bottle and get an equivalent of a 50uL dose of the
initial liquid.

Depends upon the liquid. Water is 1g per 1mL. 50mg of water would have a
volume of 50uL. Add anything to the water, or use some liquid other than
water, and the same is no longer true.

Let's just say, for example, that someone thought sage would be helpful
for improving memory (I don't know if it is, but for sake of argument...)
and they wanted to make a 30mL (milliliter) bottle of Spanish Sage oil
diluted in sunflower oil to facilitate the measuring of individual doses
containing 50uL of the sage oil. If you put 300uL (0.3mL) of the sage oil
in the 30mL bottle and then qsad the bottle to 30mL (add sufficient
quantity to make 30mL total) with sunflower oil, each teaspoon (5mL) of
the mixture in the 30mL bottle would contain 50uL of the sage oil.

I don't know if that was what you were looking for, but if it wasn't,
you'll need to be more clear.

I don't understand the first question, but the second is 50 uL = 50 mg,
that's if your speaking of water being your base, usually the mass depends
on the density of the liquid, water being 1 g/mL.

Ciao,
Marc