In this case, one could also encrypt the wallet.dat with a random key and immediately destroy it forever (so even you don't know it) with a single key pair inside and print out the private key as a paper wallet which can be kept somewhere safe (e.g. not in your house and/or next to the server hardware).
When you decide to shut the node down and spend your coins, you simply delete your wallet.dat and import the private key from the paper wallet to a freshly generated wallet.dat
That's not really going to work. Upon starting the masternode up, it's going to need to sign the input with the 1000DRK to show you own it. After that all messages will use a separate key, so the wallet can be encrypted until the next restart.
Where will node reward coins be sent?
If they go to the same address, won't the increased amount of coins disqualify that node from becoming a masternode?
In the initial message that you sign with the vin, you will sign your IP and darkcoin address to pay. It's using ECDSA signatures like the "signmessage" and "verifymessage" commands.
Also the "tickets" into the election require 1 unspent output equal of 1000DRK. We know it's unique because of the vin. Pretty amazing what you can use bitcoin for, eh?
Is it checked if your address had exactly 1000 DRK transaction once and it still has that 1000 DRK but ignores any other incoming and/or outgoing transactions?
The 1000DRK needs to go into a single input and that vin gets passed around. When you start the masternode it locks that input, so it actually can't be spent. You'd have to shutdown the client and disable the masternode to move it. If you spent it somehow, the other clients look at the memory pool for changes and disable masternodes automatically (that's the error you got from me testing).