First off, has there been enough time to assess if ShadowCash project has a working Zero Knowledge solution?
I haven't looked at the code. From the white paper it looks like it has what you are calling a "Zero Knowledge solution" based on the methods from cryptonote.
Secondly, does the Monero project of Darkcoin project have a Zero Knowledge solution?
Monero and Darkcoin are different projects.
Monero uses cryptonote, so it has what you are calling a "Zero Knowledge solution" (and has had it since day one, in April 2014). Nobody outside of the SDC community calls it that, but you can use whatever terminology you like I suppose. Cryptonote (i.e. Monero) does use a simple zero knowledge proof in its signature scheme, the same method that SDC uses.
Darkcoin uses a variation of coinjoin where inputs and outputs from various transactions are combined by a series of semi-trusted nodes called Masternodes, essentially mixing those transactions together. There is not really a special transaction type for this, just a regular bitcoin-style transaction with many inputs and outputs.
I'd like to compare anonymous transactions between the three projects being discussed here; XMR, DRK & Shadow. Could somebody post transaction IDs for each project showing an anonymous trade? I'd like to compare apples to apples when it comes to linkability and traceability. Ignoring codebase and proof differences between them for now.
Here is a Monero transaction:
http://chainradar.com/xmr/transaction/ab187945caff3a9ca5034338657808f242d8f2899fc11d93f2ef2d719bb16ecdI don't know enough about the others to provide correct links to anonymous transactions.