So i think you are looking at it from the micro view, where the concern is regarding double spending during the POS block. That is a valid concern.
But firstly, it is incorrect to say that the whole chain becomes as insecure as POS.
A chain is only as strong as the weakest link. Pithy, but true in this instance.
Rather, if you think about it, the combination actually makes the chain far more secure than POW alone.
It basically makes it impossible for a 51% miner to take over the chain unless he also happens to have 50% of all BTC.
Here is another attack angle for you, which isn't present in plain POW:
* Go slow
A majority stake holder can bring the entire chain to a crawl by withholding their POS blocks forever; so instead of a 10 minute block time, you'll have a 20 minute block time and there would be nothing anyone could do about it.
Regarding the double spending during POS blocks. POS basically works in the following form:
sha256(output hash + timestamp) = HASH
ValidProof(HASH) ~= Coin age * number of coins
So as far as POS is concerned, either you have a valid block or you don't. There is nothing further you can do about it.
Grinding is not possible for the current block.
Doesn't make any difference; there is still a constant probability of producing a block proportional to your stake, so the more stake you have, the more control you have to double spend.