You can press "Q" for slow motion without penalty.
Text guide has information regarding the level specific miscs.
Doing a LOVE1+2+3 100% run will grant all collectible-related achievements. Follow a guide closely, as some secrets are especially well hidden.
Pressing R will simply send you to the beginning of the level you're on. It won't count as a death and won't reset secret progress either. Useful if you accidentally passed a collectible or are softlocked during a longer run.
Use level select for LOVE 3 YOLO medals. When going for the YOLO medals, pause and restart after each death (or it won't count). Alternatively, tick the YOLO box before starting the level.
Cautious Completionist doesn't have to be done in Arcade Mode, just has to be a LOVE1+2+3 run that ends with ≤300 deaths. You might get it while doing the 100% run or speedrun, so don't go out of your way for this one first.
The secret ending for Freedom requires a LOVE 3 100% run, but this can be done during the LOVE1+2+3 100% run. If you got all collectibles in LOVE 3 there is a secret exit available on the last level in the bottom middle of the level. Taking the secret exit will give the secret ending and the Freedom achievement. You MUST take the secret exit or you won't get the ending/achievement.