[RPG] Does True Seeing work through Scrying

The Crystal Ball is a very rare item, this makes it powerful in the first place.

The character needs to first be attuned to the magic item in order to use it and from my understanding (on the DMG's page 159), while touching it you're able to cast scrying (with a minimum save DC of 17).

This doesn't come with any limits, you can use this as many times as you want during a day and don't need to be a spellcaster or have spell slots in order to use this (again: all you have to do is be attuned to it), you can read about this on the DMG page 141, Activating an Item: Spells.

I would say no to the spell you are quoting (which states "summon a hound of ill omen to target one creature you can see.) as this seems to go against the PHB section on "targets" for spells. Nearly all spell types or effects it must target something (odd exceptions such as teleport and sending etc) and the PHB section on spell targeting says: (emphasis mine).

To target something, you must have a clear path to it, so it can’t be behind total cover. If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

since it is highly likely that scrying wouldn't have a clear path to the target (possible if you are on a flat plane with EXTREME line of site but generally speaking)

The scrying spell only notes that you can see and hear as if you were there, it does not say you have a clear path to the target (speaking as a rules-as-written) or can act as if you were there.

Other spells depend. I would say no to long range "sight" spells such as meteor swarm as they would break the need for a clear path to the sighted target. Sending would be irrelevant as you need the same criteria for scrying anyway. Teleport would also be iffy - simply seeing somewhere does not make the location "known to you" as such. I suppose that is a DM call on what they consider "known to you" (I wouldn't consider a "common" room as "known to you", but if there were identifying parts that made it "known to you" such as a window looking onto a recognised item or location then OK you draw where you want to go and teleport into that stone wall).

An episode of the WotC podcast also clears targeting up: You always need a clear path to target a creature with a spell. You don't necessarily need to be able to see them (i.e. heavy fog wont impede) but the travel path of the spell must be clear. The spell originates from the caster so the travel path from the caster to the target must have a clear travel path. Full cover stops travel path (including glass).