Since it's possible for romanced Viconia to change her moral outlook, and for her alignment to switch to True Neutral (as far as I recall), if she is level 25+ at the time, does her Holy Symbol also change to appropriate for it, or stays an old one and so unuse'able?
(if the latter, I suppose it can be worked around by leaving her at level 24 until it happens, but it'd still be a tad annoying)

No, at least not in vanilla ToB. At the crucial time, if done right, only her alignment changes.

I believe you can get another holy symbol by dismissing and re-recruiting her, or by petrifying or imprisoning her (and then curing the ailment). This, I believe, will get you another holy symbol.

Unfortunately, this will also reset her romance, which is probably what you want.

(Does level drain + Restoration allow obtaining an extra holy symbol?)

I think the best option, if you end up with the wrong holy symbol, would be to cheat the neutral holy symbol and then drop the evil one on the ground so that it will get garbage collected.

From what I understand, the newest version of the Enhanced Edition gives her a Holy Symbol of Shar instead of the usual one; I am wondering if that symbol can be used by neutral Viconia.

Hm. I might check it after patching (didn't feel like reinstalling all mods, so have to do it yet), should be easy to do: recruit Viconia, cheat her way to level 25+ (giving her XP via console), check the holy symbol's alignment restrictions.

I have also found a "BG2 fixpack" mod, which (among other things) adjusts the symbols for the respective characters/gods (Baervan for Aerie, Shar for Viconia, etc.), if the patch won't fix it, I'll install it and see if it works.

Installed the new patch and (after fighting the mods for a while to make them work again) tested Viconia, she got the symbol of Shar, which only she can use, and which seems to have no alignment restrictions (or at least none are listed).

Edit: Using the save editor, I changed Viconia to Lawful Good for testing purposes, and she still could use the Symbol, so it's not alignment restricted anymore.