All devices are unable to be mined if an explore attempt on any device yields no files. I assume this must be a bug; it's possible to mine devices if you go and list them again.
Personal pickaxe strength just does not keep up with miner damage. There needs to be a way to make pickaxes that can either compare with miners, because being online and clicking helps, or miners need to be seriously nerfed - whether it's the cost or their ability.
I'm not exactly sure how suppressed kills are awarded, but I believe that they are given if you kill an enemy before they fire back. Alternatively, you can use the silenced assault rifle that you are given for getting 50 headshots. Any kill with that weapon will count as a suppressed kill.
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There are mute buttons in the options menu, simply press P to pause and you have a vast array of options to select :)