A blog post says this about block proposals in Casper:
A NXT-style random number generator is used to determine who can generate a block at each height; essentially, this involves taking missing block proposers as a source of entropy.
Making blocks is a process that takes place independently from everything else: validators gather transactions, and when it comes time for them to make a block, they produce one, sign it and send it out to the network.*
How many of the (up to 256) validators have the right to make a block proposal at a given height?
If my understanding is correct, during block proposal, there may be parallel competing blocks at the current height because the consensus-by-bet process has not entirely converged yet. Can a validator place his block proposal on top of any of these top level blocks? While doing so, could he choose one or several of the remaining top level blocks as uncle(s)?