How can one simulate and analyze attacks on the Bitcoin network? 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.)?
The 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...