How many full time Ethereum developers (or full time equivalents) are currently working on PoS research?
This answer is several months old but may still be useful: https://blog.ethereum.org/2015/10/18/vitaliks-research-and-ecosystem-update/
"Casper: Ethereum’s proof of stake algorithm now under development by Vlad Zamfir with help from Lucius Greg Meredith, myself and several others."
Depending on the definition of several other I am guessing there were at least 3 or 4 people spending a lot of time on it last fall. Over time more developer interest (not less) will probably focus on it.
Tendermint is not specific to Ethereum, but its PoS solution will be compatible with Ethereum. In fact, Ethereum is one of our target use-cases.