Tested build with some modifications. Didn't even need Archangel's or GA. 2/2/19 O.O

gj on the build. Gave you +1.

I'm going to have to concur with Zelkin here, Guardian isn't really necessary when it comes to Sona. She would benefit more from a archangel's.

Or DG; DG is also good. =P

Well, by the time you get the Innervating Locket, you're already at infinite mana status due to that godly change they made of it. Archangel's Staff adds a bit more to the pool, but given that you're snowballing with GA it makes sense that arch really isn't that effective with this build.

Maybe adding something like Lich Bane would help, since you're snowballing already and you could do with the extra blast of damage from your auto-attack.

Hell, that might even be an excellent idea, considering her passive. Sure, it may not sound all that awesome on paper, but with Lich Bane + Mejai's, well... you could become not just a heavy support but you'd also become a decent damage dealer.

Usually, when you play such a heavy support, getting stacks on soulstealer, the enemy team would usually want to kill you first as you can just keep giving teammates health in which guardian angel is almost neccessary. It protects your stacks and gives you a chance to run away if enemies target you with stunlock. After, GA revives you, they'll usually be targetting someone else letting you ghost, flash, or even activate zhonya's to potentially get away. Guardian Angel imo is viable on every champion basically as a final item but I agree with veil. I wrote it in the item section that veil was completely optional depending on how the game was going and whether if you stacked soulstealer, and whether if they carry some hard ccs. I agree with your point that veil might be over-kill on a more support-base champion and I'll switch it up.
I just don't feel archangel is the right item because with blue buff, even spamming stances, it's hard to run out of mana and the AP it gives is not worth it especially if you grab one late game.
I removed banshee's as it is an optional item and added DFG because it's a beast tank killer and if you get to that point of game, instagibbing someone is crucial.

I disagree with the use of Banshee's Veil and Guardian Angel as a main part of the build.

Chances are, the team isn't going to focus target you if your team is doing well with high-priority DPS leading the board (i.e. Akali or Kog'maw). It's better to further supplement your AP with something like Archangel's Staff. I think it's best to have the Veil and the GA as supplemental for if you do somehow become the object of the enemy's affection.