I can confirm, that 100% on hard can be done with saves and in multiple sessions, as in every chapter can be completed 100% on hard out of order if one wishes, but the "Pour It On" Achievement must be completed in a single session with no saves of course.

Correct. The ones that say single segment get locked out if you load a save game and get re-enabled by starting a new game. All achievements gets locked out when debug mode is enabled.

For the non-single segment ones progress is checked on a level by level basis so if you could somehow complete an episode in random order without cheating then they'll still unlock.

So I can confirm that even without saving, I did not get the single segment 100% on hard after missing some items in one particular level during the run, even though I had previously 100% that chapter on hard in a different SESSION. I have just gotten the achievement, and it is serious when it means single segment, can't turn off the game or save and quit, I sat down and 100% the whole game on hard in 4 hours with no saves, in a single session.

I also had a great experience for It's All Good, took me 4 hours and I had some deaths early on, but I messed up in 6-3 and had to die to restart so I could get to 6-7. That was quite nerve racking but it made the experience even sweeter at the end