1.
RAW it is kinda automatic,
but I've never seen it played that way even on Warhammer TV. Especially in Matched Play games because...
2. No, the points for both Blues and Brimstones are not included and must be taken from Reinforcement Points.
So if you don't have the Reinforcement Points, you can't split anyway. Do note the
FAQ, though, because it includes a very important clause:
Note that in matched play, you can add models to an existing unit of Blue Horrors even if doing so will increase it above its starting size, up until the unit contains 10 models – any excess Blue Horrors are then lost.’
This is an important change to the original rule where you couldn't ever take them above the starting number of models in the Blue/Brimstone unit (e.g., if you lost 2 Pinks the cap on the Blues would've always been 4 models).