What is the best way to give fiscally irresponsible parents allowance?

Topping up their CPF SA account would be a good start, its locked in and the payout wont start till 70 unless you request it anytime past 65 years old.

For insurance, cheap and good coverage dont go hand in hand, rather when you metioned good coverage, what do you define as good coverage. Take a Personal Accident plan, there are numerous PAs and priced differently, for the reason that they have different coverage of "accidents" and per accident limit.

I would ask myself, do i want to get insurance for them for my peace of mind or i want an inusurance plan thats the most comprehensive. ?

But if you have spare time horizon, insurers run marketing events once a few months, look out and get them then. Its more "bang for buck"

I bought my parents insurance (one from AVIVA, one from AIA).

With regards to allowance, I instead buy home groceries and toiletries - because if I give in cash, it might go to the manicure shop O_O

Unless they are in pink health, if not it would be very costly. Maybe can help them to create an investment account and put the money inside there.

I think CPF top up is ideal since you mentioned "fiscally irresponsible parents". I would think that it's important to make sure there's forced savings for them, so that they don't anyhow spend away the allowances provided by you. Furthermore, CPF RA provides 4 to 6% interest, if one's combined CPF has not hit $30k, the interest earned would be 6%. You might want to consider this avenue.