Vit. C dissolves much better in warm water, but it also dissolves much better when the crystals have been ground into a fine powder (due to the extremely enlarged surface area).
We bought a special grinder (480 dollars excl. shipping and taxes..) and we now sell ultra-fine vit. C powder ground with that grinder.
I have never seen brown discoloration with vit. C dissolved in water and I presume this is caused by using water with a lot of dissolved minerals ("hard" water).
I don't think oxidized vit. C has a noticeable color, so it may be that the acidic vit. C combined with a Mg or Ca to form a visible compound.
Vit. C does degrade by Oxygen, so a good idea would be to take a pan, boil some water, let it cool until it is still warm and then dissolve the fine vit. C powder into it.
Boiling the water will expel all dissolved gasses, including O2.
If you have hard water where you live, use demineralized water. This is cheaper than distilled water and just as good. Demineralized water is water suitable for use in old-style car batteries and steam irons. It's sold in pharmacies and car accessory shops.
Vit. E does not dissolve in water (but in oil) so it can't be mixed with water unless you use an emulsifier. So no, you can't mix them.
As to Ferulic acid - I really don't know..