adjectives - Confusion about 'less' and 'fewer' in sentences with countable/uncountable nouns

1. There were no less (or no fewer) than fifty persons in the dining hall.


2. In 25 words or fewer/less, please summarize what took place.


3. fewer / less calories?


4. The hamburgers should contain no less/fewer than 50% meat.


5. Less/fewer than five percent of the population will be affected.

We use "fewer" for countable noun and "less" for both countable and uncountable noun.
My confusion is that can fewer and less are interchangeable in all these sentences (or in that kind of sentences)? I also read that both "fewer" and "less" are acceptable for countable noun. But if we follow strict grammar rule what should be answer for use of "fewer" and "less"?

Note: Cambridge Advanced Learner's Dictionary say this is traditionally considered bad English to use "less" for countable noun.