About the extra ropes, I haven't tested it on slaves purchanced from slavers but that behaviour sounds about right. The game just assumes that the new slaves being returned to the mansion have ropes on them and puts the rope back into your backpack. Sort of makes sense that the slavers give you a free rope with your purchace.
With the double mana message, I've also noticed that myself and was never quite sure either. I wasn't able to get it to happen reliably and it didn't bother me enough to really look into it. If you care and you are able to come up with steps to get it to happen more reliably I could look into it some more.