I don’t think that increasing IBI would make any difference. The toxic bank is looking for a way to minimise losses, so the bank would not want to pay additional IBI. Absent and insolvent owners would not be paying either, so the chances of collecting the additional tax are probably close to zero.
As for compulsory purchase and demolition by the local Town Hall, do you think that local residents would agree to pay additional taxes to cover the cost of this ?
I would guess that nothing is going to happen to these unfinished properties, and nobody is going to do anything about them until it makes economic sense to do so. Given the current state of the property market in Spain, I would not hold my breath waiting for action in this matter.