Nice security research. Can this attack be made profitable, or is it just DoS?

This "attack" is a nuisance just like the HighS malleability.

Is fixing of this issue requiring a full 'hard forking'

No. As trout already wrote the miners can just take a higher fee for transactions with a large number of SIGOPS.

1) You are missing that miners are interested in fees. They have a right to include/exclude any transaction.

They might be interested in the BTC value, too. So it's interesting for them to include all transactions to preserve the value of their BTC.

So this will be able to have a fix but requires convincing of the largest pools.