Still, in prescriptive usage, it should be "slowly" not "slow".
Would you point to a grammar book prescribing it?
I think the main English sentence should be "Don't go so slowly". Pomalu = adverb = slowly.
This is discussed above. It is fine.
There are not any discussed.
Then just google "slow slowly". http://www.learnersdictionary.com/qa/Slow-Slowly-and-Flat-Adverbs
Doesn't a negative imperative require use of imperfect verb here... "nejezdi"?
Not strictly, but it will probably be more common.