Are there any other platforms or tools beside TestNet where I can simulate and analyze various attacks like double spending (finney/block withholding attacks etc.)?
regtest operational mode of
bitcoind is not as similar to
testnet as you might think. The
regtest mode offers you a totally local Bitcoin blockchain which is easily controlled by your local
bitcoind with low difficulty. According to this post, it is furthermore possible to build a network out of multiple local
bitcoind instances, such that you have a local Bitcoin network of your own.
You can control this network freely, but you can only simulate behavior that
bitcoind allows you to assume. I am not aware of whether or not this is sufficient for your cases. If not, you could maybe try to fork
bitcoind and somehow add the behavior you want to achieve...