1) I can see by default Security Margin is set to 2% (I remember it used to be set to 60% earlier). At 2% that doesn't leaves a lot of room for the coin to move. Don't you think it might result in a lot of sells at 2% loss ?
Use 2% only when you try to scalp coins like BBR, PINK, PASC or any other coin that can go +200% now and -500% in one hour: dare an higher GAIN on those coins too, like 3 or 4 %All other coins i have security margin disabled (60%)
2) Is Max latest prices the number of 15 mins candles ? ie 300 = 75 hours of data ? Or is it the number of different tickers received (thus unknown time period) ?
it is the number of different tickers received you set in Time Delay Price3) Could you store our credentials locally so we don't have to input them manually each time we start Gunbot ?
No 
Just leave the configurator open and change coin pairs then press the start gunbot button again: it will not ask again for your credentials.