In these uses, the two words have the same meaning.
Student or Learner
1) Are the adjective "criminal" and the noun "criminals" used correctly?
a) Their acts were condemned/ denounced as criminal.
b) He condemned/denounced their acts as criminal.
c) The two men were condemned/ denounced as criminals.
d)He condemned/ denounced the two men as criminals.
2) Furthermore, I would like to know whether both "condemn" and "denounce" could work when used with this word in more suitable contexts even though they have different meanings.
Yes to all your questions. You've got it!