You're overthinking this. A tax-free scholarship or grant that is used for qualified expenses, as defined in pub. 970, can be used to negate the 10% penalty on a taxable 529 distribution, within the limitations described in the pub.
A Pell grant is not a fellowship grant, but that doesn't mean that it (or any other need-based grant used for qualified expenses) can't be taken into account when off-setting the 10% penalty.
Yes, within the limitations outlined in pub. 970.
I guess the worst case is that the IRS comes back and assesses the 10% penalty on taxable 529 distributions, along with interest and late payment penalties. If you're worried about this possibility, then then maybe you should just pay the 10% penalty on all taxable 529 distributions.