"If someone gets 100 (or a bit more) backers, that means 200+ didn't back him/her. How is that a recipe for much-needed unity? "
The 100+ backers thing is to limit the number of candidates.
It is also likely to encourage possible candidates to unite under one of them, as otherwise you get a situation where a lot of them couldn't even get to the first stage. So thereis bound, as I have said before, to be a sort of "pre-election" election to make sure there are at least two candidates.
Unless of course only one manages to get 100+. But I am betting that if Boris Johnson has another go and looks like getting to 100, his opponents will try to find one person to stop him being the only one.