I'm working on decentralized (no reference nodes) masternodes and instant transactions. I hope that by the end of this month or sooner we will have public testnet with working masternodes and instant transactions, all code will be open sourced by that moment. DarkSend is much more complicated subject, don't expect it soon.

Amount for running masternode hasn't been set yet. It depends on how many masternodes we want which is not yet clear. The more is better for decentralization, on the other hand we need not to bloat the network with masternode related messages. Instead of setting the required amount it may be better to set the maximum number of masternodes and minimum required amount. Once the number of masternodes will reach the maximum only masternodes with maximum deposits will remain and others will be automatically delisted.

I was thinking about it. The problem is that given only address you can not encrypt message such that only owner of the address will be able to decrypt it, address is only suitable for checking signatures. Thus address owner should broadcast some additional information to be able to receive messages, I believe Bitmessage is working this way so it is possible. Well, as you know, I'm working on other things right now