My first thought is to make a Necropolitan, so you'd get 1d12 hp/level plus your Cha bonus. It could also qualify for and take some (physically) strong Gish classes such as Abjurant Champion and Eldritch Knight. I could see this being used for a spectacular Glaivelock build. As for your questions specifically:

1. Possibly, though I really like the mechanics of the delay between spells. It looks similar to the Arcane Swordsage variant, which is broken. Especially outside of combat when round-by-round actions don't really matter it could be extremely powerful. For example, spells I'd get with an Arcane Swordsage such as Greater Mage Hand, Greater Dispel Magic, Disintegrate, Ruby Ray of Reversal, and Iceberg from Frostburn can make a huge difference outside of combat. If a character doesn't have to worry about running out of spells, they can just spam those all day and just brute force their way through noncombat challenges. Summon Monster spells would be extremely potent and really drag the game down, especially if the character opens every fight with the highest level summon monster spell they can cast. Maybe restrict it to certain schools of magic rather than allowing it access to the entire spell list.

2. Its versatility is dependent on how many spells a given character knows, which could get extremely high. Basing its spell DCs and other abilities on Cha and spells known and highest level spells on Int is a good way of balancing that, but the character could always dump other stats. It can get the same spells as a Sorcerer, and can know more of them, so I'd put it no lower than Tier 2 for raw power, and possibly among the Tier 1 classes based on spell selection. If used in a strong Gish build it could make a capable combatant as well as a potent spellcaster, though it would lose many level-based abilities including how many buffs it could have going at a time.

3. I think it's a fairly even match, though this class would require more thought for spell selection than the ToB classes need to be powerful, and this class has a lot more potential in the higher levels. Going back to what I said earlier, it's very similar to the Arcane Swordsage variant, though that can spam a given spell every other round regardless of level. I think the two are fairly even though, which means it's quite a bit more powerful than the other ToB classes including the standard Swordsage.

4. Maybe a Glaivelock build, using the Eldritch Glaive invocation found in Dragon Magic. It depends on what "+1 level of existing spellcasting class" prestige classes would advance. I'd expect it to advance your spellcasting, invocations, and eldritch blast, just as they would for a Warlock or another spellcasting class. It looks like you're trying to use a low HD to balance its power, but Gish classes like Abjurant Champion and Eldritch Knight or even Divine Oracle can mitigate this, plus an undead race such as Necropolitan would be a decent choice. I'm also wondering if this class would be considered a 'spontaneous' spellcasting class, for prestige classes like Ultimate Magus. The Sustain Dweomer mechanic is a good reason to stay single-classed, but using a dual-progression prestige class can get around that by relying on traditional spellcasting for buffs and spamming offensive spells with this. Something like Magister 5/ Mindbender 1/ Ur-Priest 2/ Mystic Theurge 8/ whatever 4 would be decent, but somewhat MAD depending on how high you want Int to be.