Will you be able to play the PoE1 Campaign with a PoE2 character?

So I'm a new player and am just catching myself up with the PoE2 news. Got to say I'm incredibly excited for the new graphics, characters, gem system and the ability to have all my auras activate with 1 slot. And of course, everything else. I think I understand the majority of it but do have a question to clarify some misunderstandings on my part.

So in PoE2 when I go to create a new character, if I choose a PoE2 character (one that was added in PoE2) can I play the PoE1 campaign with him/her? And vice-versa with a PoE1 character playing the PoE2 campaign? I'm guessing that you would be able to? And if not, how would this work? I know PoE1 and PoE2 are the same game and PoE2 is an expansion. As in PoE2 is not separate. It's fundamentally just PoE. But I'm not clear on how the characters and campaigns work with eachother. But thought I'd ask nonetheless.

As well, in general, are there any downsides (from any perspective) to PoE2 and it's changes that are actually warranted and not just misunderstandings? Like will I lose access to any items or anything like that? Looking through the forums, it looks like a lot of people are/were worrying and thinking X is going to happen when in reality, X is not going to happen. Thought I'd ask to get a clear answer vs wading through all the BS. Thanks guys and gals!

Quite sure the new char will only be for poe2 but end game is shared. But should probably wait until they reveal more

POE 1 and 2 will have shared client, but they will remain as separate campaigns, because POE 2 is placed 20 years after POE 1 campaign ERA.

You will be able to keep all your MTX and use them in POE 1 and POE 2.