a) "Regarded" – the simple past – is probably well understood. If, though, you are referring to a philosopher or school of philosophy that was no longer in existence by the time the romantics appeared, then "had regarded" might be used.
b) This is difficult. If you say, "they were," of course the phrase refers to the plural "feelings." However, this implies several distinct feelings. The romantics are really thinking about the abstract idea of feelings as the human essence, and "it was" refers to "essence."
I think the sentence would be better and parallel and not jarring if it were this:
"¢ Whereas the philosophers regarded the feelings as an obstacle to clear thinking, the romantics regarded the feelings as the human essence.