Ethereum POS is already running on a test net, you're welcome to go over there and see what happens when you try to use your stake on "many forks at the same time". Spoiler: you won't be able to do so without losing your stake very quickly.
Well not quite. The test net is actually a hybrid of proof of work and proof of stake [1]. Proof of work is used as the main decider in the case of a fork, however if long forks are seen then checkpoints validated by proof of stake are used.
The slashing function is in the contract [2] but there is actually no incentive to duplicate votes on the testnet since there is only rewards for voting (block rewards are given to proof of work miners).
Don't totally follow your argument, but if you're saying that we won't have a full understanding of the POS economic guarantees until we have empirical evidence from a "live network" with significant rewards at stake I agree 100%.
