I went into this video expecting to be disagreeing with you on a lot of things, but to my surprise, I agree with you on almost everything.
I completely agree with the way you ordered the games: 2, 3 and 1. The first game definitely has some minor problems when compared to the sequels. Level design isn't as good, some hidden areas require guess work and leaps of faith, the boss battles are simple, and a general lack of content compared to the sequels.
However, this is to be expected. I feel it's worth noting, but not worth dwelling on. To be fair, you did mention several times that, despite all the bad things you had to say about the first game, that it's still a fair game. It's just that I find it odd that so many people will say this about DKC1, but not about (for example) Super Mario Bros 1. The first SMB game is in the same boat as DKC1 as far as the sequels go (SMB3 and World). Yet I've never heard anyone complain about SMB1's level design and boss battles when compared to its sequels. It's almost unfair to compare the first game in a series to its sequels. OF COURSE the sequels are better, there would be a problem if they weren't.
Pretty much everything you mentioned that's "off" with DKC3, you're right in line with a lot of people. I never had a problem with it, personally. Maybe it's because I was one of the few people that actually played DKC3 back when it first came out, maybe I never really looked at it objectively enough? Levels like Poisonous Pipeline, I never thought, "This stupid game! They've gone too far!" I thought, "Wow, this is kinda hard. OK, let's do this!" I'm glad you mentioned the more open design of the over world map. That's probably my favorite part of DKC3.
I've watched them in my adult life, and I can tell you, they're absolutely terrible. But they're the kind of terrible, when you know a kid made it, they're pretty humorous.
