Honestly I think you are to blame for this one -- I wouldn't give any compensation. You did ask for the IRROPs re-booking to UA which is likely to sink you for compensation.

Once you gave the DL agent approval to switch carriers, I don't think you can ever view this as unapproved re-booking? You authorized her to switch you to 11am, she wasn't able to, your seats on the original flight were gone (due to IRROPs someone else probably grabbed either the whole thing or the relevant inventory for ORD - CVG or CVG - BOS), and then she managed to get you on the next flight that could get you to BOS (which was UA since DL didn't have anything available).

If you hadn't asked for the UA flight, the whole thing would be different (and that's how I originally read your OP but upon re-reading I saw that you asked for the switch to UA).

You live and you learn but in the same scenario, where weather is causing major issues, you don't ask to switch carriers. There is too much uncertainty, especially when flying from a major UA hub, the likelihood of those seats not confirming was close to 100%. Why would UA ever give those seats to DL when they are dealing with their own major IRROPs due to the same weather?