This explains some odd behavior but it's not a big deal.
This is a monstrous fuckup of enormous proportions, if this is true then their trading engine is broken beyond repair and should be taken down and replaced by something else immediately!
Right, out of order execution is not acceptable.
I dunno. It just depends on the matching algorithm. It may not be industry best practices but I don't see that it necessarily shouldn't be acceptable in and of itself.
Read again. If what it's written it's true, then their trading engine is exploitable - an exploitable trading engine is NOT acceptable, it's a SCAM