I've tried to focus only on economical aspect of the network and general conditions that would make underlying hardware sustainable. Currently system thrives on love for science and I also think this part is awesome and should be cultivated.
ad) To adjust to these machines working in their previously idle time, we just need to factor dedicated machine income requirements. If there is 10 pcs with specs equal to our reference computer, each working 2.4 hours a day on BOINC, they equal together one reference computer, thus each should earn 1/10th of the latter. So basically such 10 computers are represented by one reference, dedicated computer in the model above.
ad2) Yes, provides a rebate on power costs, but for how long? Without current crypto bubble, who knows, GRC could be 50 times cheaper and rebate would be quite negligible.
I don't think this would ever happen. Fifty times cheaper would put GRC at USD$0.0006, and people would buy stacks of it far before that point.