I am currently on my second playthrough. I have received Soulseal Staff quest, and I need Angel Beads for it. The problem is, I have none right now, and have done both Elizasleyn and Principatus in Melodiana's trial. Can I get the beads anywhere else, or is the quest uncompleteable for this playthrough?

Chapter 8 of Serawi route right now (ready to advance to ch.9, but that will fail the quest). Characters are lvl 50-60, so EX maps are out of question.

You can get Angel Beads from Tainted Angel in Yuidora Mine: Isolated Battlefield and Room of Legensaad, but those are probably chapter 9.

During Hannah's second escort mission, I think I saw a Principatus(-looking thingy) during "enemy turn phase" (i.e. among the portraits of enemy units that appear in bottom-left corner during their turns). Is it really there? (I didn't do the mission, just went in to look at the map)

Angel Beads
Elizasleyn
Principatus - Hallowed Ground (Cliffside Church)
Voice of God - ?? (Cliffside Church), Blockaded Pit (Yuidora Mine)
Tainted Angel - Isolated Battlefield (Lv 80 Yuidora Mine), Room of Legensaad (Lv 95 Yuidora Mine)

The Principatus at the Cliffside Church is in Trial at the Cliff for Quest 53 (Ch7), so you've missed that.
Not sure about the Voice of God that's listed at the Cliffside Church in the Monster dictionary.

Your best choice for Angel Beads is Blockaded Pit (Lv 70, Yuidora Mine).
That map should be opened when you do Untouchable Cage (Lv 70, Yuidora Mine).
You can do that before Chapter 9, there are a few Voice of God's there.

You probably saw a Glou Harpua in Mountain Path (Cliffside Church) which is similar to a Principatus, different coloured hair.

Thank you, I'll try that.

I meant the Misanshel escort mission, not Cliffside Church one; and I'm pretty sure it was a pink-haired angel. The wiki says that Hand of God, Blade of God, and Guardian Angel are present in Misanshel, so it could be one of those (don't know how they look).

I did get the beads from the quest Principatus, but spent them on crafting a bunch of items that I won't be able to use for quite a while.

I see, you mean Eternal Sanctuary (Misanshel).
The first time you go there, which is the escort mission, there are a few Guardian Angel's.
They have pink hair but they don't drop anything.