IM was holding back, this was not Superman fight that's why IM didn't rip Superman in half like the rest of the New God's. If IM wanted too he could have one shot Superman back to Earth, IM was toying with him.
IM took 3 punches from Superman, as Superman said he could shatter planets (indicating he wasn't holding anything back while punching IM). After 3 punches, without a single cry of pain, IM stopped f'ing around and trapped Superman. Prior to that, all of IM's attacks hurt Superman.
Drax also ripped out the Magus' heart that doesn't mean he easily rips out Surfer's heart. Im is also powered by the same source which powers the new gods. No, for two reasons. Making a sound when you are physically pushed back is kind of expected. The dialogue makes it clear eventually these elite top tier attacks will bring him down.