For Harry, his friends like Ron and Hermione are likely to be his allies. They've always been by his side through thick and thin in the original series, so it's natural for them to support Harry in a confrontation with Dumbledore. Neville could also be an ally, given his growth and courage shown in the books.
For Lily Potter, her husband James Potter might be a likely ally. He would support her in any conflict she has with Dumbledore. Also, her close friends from her school days could side with her. They know her well and would trust her judgment.
There's also a chance that Dumbledore gets caught by a group of rogue magical creatures. For instance, if he was exploring an area that was supposed to be off - limits to wizards and the creatures there saw him as an intruder. They might not be as rational as wizards and could hold him captive without really understanding who he is.
Well, it could be that Dumbledore has some sort of hidden device or spell that he activates when kidnapped. For example, he could have a charmed object that sends a signal to his allies, like the Order of the Phoenix. His allies then come to his rescue and he manages to escape during the chaos. Also, Dumbledore is extremely intelligent. He might outwit Grindelwald by engaging him in a complex conversation full of riddles and misdirections, which gives him an opportunity to break free.
It's really hard to say. Dumbledore is known for his vast magical knowledge and powerful spells. However, Lily Potter was also a very talented witch. If we consider her love - based magic, like the protection she cast on Harry, it could be extremely powerful. In a direct magical duel, Dumbledore might have the upper hand in terms of experience and a wider range of spells, but Lily's love - fueled magic could be unpredictable and potentially very strong.
In this fanfiction, it could be about a philosophical difference. Lily Potter might have a different view on how to deal with the threat of Voldemort. Dumbledore is often very strategic and calculating, while Lily could be more impulsive and driven by her emotions and motherly love. So, they could be at odds over how to train Harry or which allies to trust in the fight against the Dark Lord.
One interesting plotline could be a magical duel between them where they both use unique spells not seen in the original books. Maybe Dumbledore is under the influence of a dark spell, and Harry has to fight him to break the curse.
It could be Severus Snape. He has a complex relationship with Dumbledore. Maybe Snape discovers some of Dumbledore's plans that he doesn't agree with regarding Harry Potter. He then sends a Howler in a fit of anger to express his disapproval.
In a fanfiction, Dumbledore might steal out of desperation. Suppose there was a situation where a curse was about to be unleashed on Hogwarts, and the only way to stop it was by stealing a particular ingredient for an antidote from a place where it was not supposed to be taken. Dumbledore, seeing no other option, decides to steal it. His motive here is to save the school and its students, even if it means breaking the rules.
Fred and George Weasley are very likely candidates. They are known for their love of pranks and their mischievous nature. They might see pranking Dumbledore as the ultimate challenge.