[1] Because there's more money and the same amount (or less) of things to buy. Think of it a bit like someone issuing stock: yes, that dilutes your shares.
[2] It's not just pay. You CANNOT pay me to do that job again. I'm sick of being cut by glass. I don't like dealing with horrific bloody injuries, though I'm incredibly conflicted there--I mean, I did volunteer for it and I don't regret doing it and helping people. And I'm glad to be able to do it and that I was able to help people and I'd even volunteer again... but it's a horrible thing to have to do and I don't wand more of it.
Basic income is just welfare on steroids. The effect should be something that depends on the scale of how much money and how many people have their 'stake' diluted by how much. The scale of this is bigger than before (everyone gets money) according to what's planned--I don't seriously think anyone will implement it as $5 for everybody and if they did, it shouldn't have any meaningful effect.
Due to more money being issued, there should be more inflation caused by it, as we can't all suddenly start producing more windows and whatnot just because we're paid more. People are also likely to start charging more because everything costs them more and people have more money to spend now.
Now, all that said, maybe it could work as a replacement for welfare. I'm not sure about that, but it doesn't seem terribly unreasonable. Maybe everyone has an ID card and they can charge $X a month for food/rent/clothing, especially with the theory that people who don't actually need it might not use it. As I said, the main issue is the scale. If you get to the "work is optional" state that everyone seems to want, that's where I worry about the effects. But, there's obviously a long continuum between it being 0% optional and 100% optional and I honestly don't know what things happen where.