Am I eligible for miles on the tax portion paid when I redeem my flights with miles?

Yes!

Yes, although charging the taxes to DBS Altitude will be better as you earn 3mpd compared to 2mpd.

Cheers.

Yes, the tax portion is eligible for miles rewards.

Yes. Expenditure only doesn't award points, etc when you are spending on certain categories such as insurance, money transfers, etc.

If you are paying for a product or a service, you will generally be eligible for rewards.

In this case, taxes are payable as the airport renders services for you when you board your flight, such as security.