Chances are that someone offering to pay chunks off their mortgage is good for 100% of the repayment, so you certainly shouldn't expect any default probability to be built in to a pay down.
NAMA focussed on high worth individuals for a reason, the administration of going through hundreds of thousands of mortgages to assess who's good for repaying and who's not would be very labour intensive and an absolute neccessity (and practically impossible) in making an assessment of economic value.
As for the cost of funding issue, I expect the banks fully understand that in the short term trackers will be loss making simply because of the cost of accessing funds. In the medium to long term, however, they would probably have an expectation of accessing cheaper credit, perhaps by increasing their deposit base and keeping deposit interest low as the ECB increases the interest rate again. So I would expect that the banks believe most trackers still have a positive economic value over longer terms.
That said, no harm to keep asking.