What about Merlin? Didn't he attend Hogwarts too?

 

No evidence in canon that Merlin was considered a particularly powerful wizard. From what JKR has said in interviews, he seems to be remembered primarily for his social/political ideas and leadership (namely, magical and Muggle cooperation) rather than any magical feats.

 

Dumbledore himself described Voldemort as the best student to ever go through Hogwarts, and Riddle came after Merlin, this means Voldemort was better than Merlin.

But as Taure said, Merlin was recognized for things other than his magical abilities.

 

Dumbledore wasn´t their at Merlin´s time or that of some other students so he doesn´t know for certain, besides I think he himself was better than Tom he was published as a teen and imo the superior wizard he was just being humble

 

Can you cast a Cheering Charm on yourself to make casting the Patronus easier?

 

True, however we know magic has advanced since then in terms of spells and potions. So in terms of that, Voldemort would win.

 

I don't think there is any evidence in canon of this. If we try to get there logically using the information in canon, well, we know that a cheering charm makes you happier, while dementors suck positive feelings out of an area. A patronus requires a really happy memory. Now feeling happy doesn't really equal relieving a happy memory (as is canonically required for a patronus), so we can say that it shouldn't directly allow you to do so. On the other hand, feeling happier does allow you to better remember happy memories, so you have that too.

However, it might be counter productive when facing large numbers of them, because they get attracted to large amounts of happiness/excitement (a la Quidditch Match in third year). So a cheering charm should have the effect of making it easier to conjure up a happy memory while giving you less time to actually counter a dementor, given that it gets even more strongly attracted to you. The closer a dementor and the more it focuses on you, the harder hit you are by its effects. So I'd say that there's a critical distance between you and the dementor where this has no effect. If the dementor is closer than this distance, casting a cheering charm is counter-productive. If it's further away, then it will make it easier to conjure a patronus.

 

Do we know hi old the Marauders were when they became animagus?

 

Old enough to know better, young enough not to care.

But really, human transfiguration is NEWT level, taught at 6th year and up. They could have done it earlier, which feels like something Jo would have done, but it would have been studying ahead.

 

What about that Prince of Enchanters stuff on HP wiki (I know a terrible source but I think the info is straight out of Pottermore) if it isn´t wrong than I guess he was pretty powerful

 

They figured out Lupin was a werewolf partway through first year. McGonagall demonstrates her animagus transformation at the beginning of 3rd year (not 1st as in the movies), and they worked on it from then until they got it mastered in 5th year- even Pettigrew.

 

Was it just in the films where Harry's and Ron's voices didn't change when they imbibed Polyjuice back in their 2nd year to pose as Crabbe and Goyle?

 

Yeah that's in the films only, in the books the voices change.

 

In which book and in which scene did Dumbledore wear a flowered bonnet?

 

In the first book, during Christmas.

 

Does a metamorphmagus need to work out, or can they just shapeshift their body to be fit when they need to?

 

There is no canon evidence of this, but it stands to reason that if you can shift your body into any form you should be able to enhance yourself to be fit.

 

I think there's no question that they could shift to appear fit, but there's probably some sort of limit to the health benefits of being a metamorphmagus — Tonks being an unaging regenerator is the sort of thing you'd expect to come up at some point — so it may be that some of the more functional benefits do still require exercise.

 

Were the Dementors guarding Hogwarts in PoA used because Sirius was would have been recognised by/particularly vulnerable to them after his escape from Azkaban? Or was it just because?

 

I don't think that there's any canonical evidence about that, but iirc, the dementors were furious about the escape. That might have been one reason. Another might have been that they seem to be a pretty foolproof way to guard an area. The students themselves were given a long warning in the beginning of PoA by Dumbledore that dementors will not listen to excuses, blah blah if they catch a student out of bounds.

Logically, imho, dementors were trusted to guard the most maximum security prison of wizard-kind. The point wasn't to use dementors to catch Sirius, it was to guard Hogwarts's perimeter. Dementors were a tried and tested method of doing that. The government probably couldn't think of a better method to do it.