2) Considering he's devoted the last 15 years of his life to destroying the Sith, I'm pretty confident he would have learned everything there is to know about Vader's abilities.
Now we’re entering the realm of speculation and guesswork. Just as much as much as Maul’s adventures have been kept unknown, we can only make logical guesses to what he’s been doing. It is true that theories are fine when discerning the unknown, but in judging the validity of hypotheses from an observation, there exists scientific methods that allow us to determine from a list of possible choices, which theories are the most plausible ones, and which are not.
For Maul to have learned everything there is to know about Vader's abilities, you’d need to introduce a sub-plot where he’s spends a peculiar amount of time spying on the latter. Which is a less reasonable conclusion than Maul, like many others, simply having knowledge that Vader is a famed Jedi hunter. Unreasonable explanations for a phenomena can be discarded in favor or more defensible ones. For example:
Something really odd happened at my place last night. I left my unfinished bowl of cereal and milk out on my deck before I went to bed. This morning when I got up, all the milk was gone. Well, all summer I’ve had these rings of mushrooms popping up in my backyard. I’ve read that those mushrooms grow where the fairies dance around in circles at night. I think that the fairies that were in my backyard came up and drank the milk. I’m going to write to the newspaper and tell them I have proof that fairies have been in my backyard.
-DarthChrisHemsworth
This is why entities such as ghosts, UFO's, and strange creatures are so popular. These are things that science has to deal with, in maintaining people's respect for a predictable, natural world. Being able to recognize the least plausible hypothesis and explain why it should be abandoned in favor of a reasonable one, is an important part of critical thinking.
So what Maul's become humble in his abilities now? Really, you think he's matured or something.
No, that is not what I’ve concluded. I am suggesting first and foremost, that Maul is not being genuine, and that if he is, his genuine word does not have to be the objective truth.
Actually Ahsoka has encountered Maul before
She has?
and obviously knows Vader.
Correction, she knew of Vader. She knew Anakin.
And yes when there are several people around who don't object, especially people who know something about the subject matter, it's usually a further sign of the statement being true.
That’s incorrect. This is an informal fallacy known as the Argument from Silence. Their lack of response, or in this case, lack of contradiction, is not evidence that they agree with his reasoning. It can not be deemed as such. For starters, their silence can be interpreted as ignorance on the matter, given that they’ve only seen a mere minute of Maul’s fighting; in which he was likely holding back.
As a secondary point, I can just point to their lack of unison-agreement, as some evidence against Maul’s words. Nobody makes an overt gesture to agree with him. Nobody, for example, interjects with a ”yes Maul, you’d get stomped” and we know that both Ashoka & Kanan remain weary of his true intentions.
For example when the Chancellor says "This Republic which has stood strong for a thousand years" in front of many other people, with no one correcting him, it's pretty safe to take it as fact that the Republic has been around for a thousand years.
Muddy example, and I must say that you’ve picked a rather poor character to use as proof for this occasion. In fact, you’ve likely picked the worst character in the franchise (possibly in the realm of fiction too) for relaying objective truths. I could just mention this statement he made in front of anakin:
"I haven't ran since I was a boy on Naboo."
-Darth Sidious
Anakin didn’t contradict this statement, so is it now a stone-carved fact? Another another truth from Chancellor Palpatine, also known as Darth Sidious, who lived a double life of a Naboo Senator just to manipulate the political system of the Galactic Republic until he was named Supreme Chancellor, and eventually Emperor? Perhaps not. Perhaps Palpatine, much like Maul, was attempting to deceive his contemporaries with a statement that isn’t totally genuine.