I believe bitcoind is very DDOS resistant and so is Darkcoin as a consequence. Not sure about monero however. 
4. and 5. (no clue how this relates to 5) - collusion on its own is irrelevant. Of course it will be ineffectual. It would always be coupled with the compromise or the DDoSing of additional "masternodes". In such an event, the number of available masternodes is massively reduced, which means the the little Google spreadsheet is incorrect. Many hosts will null-route a dedicated server (even more so a VPS) if it is experiencing a DDoS. Other attacks such as a DDoS group constantly requesting a stream of blocks from a masternode would lead to its traffic exceeding normal use and massive traffic bills. Similarly, a DDoS group could broadcast thousands of bad blocks a second to a masternode, leading to it chewing through CPU cycles doing very heavy X11 verifications, ultimately leading to it becoming inaccessible or being killed by the oomkiller. Those are just off the top of my head, of course, and the attack stands regardless of whether these specific scenarios can be mitigated against.
Refreshing to read opinions of someone who actually has a clue instead of crypto noobs hyping their vapor buzzwords. I consider myself to be crypto noob as well, however:
What if the Darkcoin wallet was monitoring the state of the network, and if the running masternode count fell for example 50% in a short period of time, it would stop denominating funds (denomination is the anonymization process which runs in the background). This wouldn't affect people sending or receiving coins because they already have them denominated/anonymized. It would only prevent people from re-sending received funds during this ddos period.
Every other coin has to use nodes as well, and they can be ddos attacked which can halt the network for the duration of the attack. It's not only Darkcoin that is susceptible to ddos.
Masternodes do provide a nice easily identified target - I think some effort on testnet is needed at some point soon, maybe after RC4 release, to try and break testnet MNs with such attacks and figure out what we can implement to mitigate them, because we can't stop them.
I don't know a lot about it, maybe we have a white hat among us who can assist?
