I think you are over selling the uselessness of this creature. Daylight powerlessness is indeed terrible. In natural daylight. So you character will only be outside at night, and when your in underground areas and areas specifically built to accompany creatures that don't like daylight, of which there are many.
As an undead creature you are likely to already be in a group with undead and/or necromancers which will understand your weakness and may even share it. Such as a wraith, a shadow, or even to a lesser extent the mighty vampire.
The draining melody. To cease your performance and offer a new save, you must stop performing for a full round. That means more than six seconds. Your character will fully understand the limitations and drawbacks of your own ability. It is simple to stop a bard song and start a new one within the timeframe, granting no new saving throws. It is simple to hum quietly when you aren't saying anything, casting anything, or otherwise performing to continue the saving throw the party already made. And that's if your party isn't straight immune to your ability as you say half of creatures are.
Finally, your ability says on the round after they initially fail. Your whole ability is not enthrall. That would be like building your whole character around the secondary effect of a poison. Your main ability is a save or daze with riders. That you can do passively. That you can restart if you absolutely have to. This effect is a supernatural sonic mind affecting compulsion. Sonic: deaf creatures are immune. Mind affecting: mindless creatures, undead, and constructs are immune. Compulsion: a first level spell grants immunity. Your party will be immune, undead, constructs, and mindless creatures are immune. So by extent, anything that is immune to a beguiler, a t3 class is immune to your abilities. I don't see this as locking the race out. Because past your seemingly only ability is incorporealness, high charisma, good dexterity, decent skills, and immunity to most affects that aren't just damage.

Yes, compare this to the ghost template and it looks like crap. The ghost template is effectively on ur-priest levels compared to our beguiler. Because it has no lost hd. It doesn't have the drawback of sunlight. And it has rejuvenation. I think I can count on one hand the amount of undead templates that will be better than it.

So it doesn't deserve la +3. Does it deserve la+1? That's what we are here for.