Story-wise? A lot. Whole T͖̲̬h͇̜̭͈e͇̠̬ ̶̠O͚͓̼͚̲̙̭̕l̼͎͜d͇̺̬͡ ͎̩̲̮̜̭͝O̹̱͢n̵̬̬̲̦͈e arc. And an opportunity to fight Hyakki Oni.
Scene-wise? Nothing.
Again, only some story pieces. Not so much as in Genocide route tho.
There are five variations of Neutral ending depending on your actions.
Also, you may want to marry each girl to see their extended ending.
Your guess is wrong - all the book (96% for the current version, the game is still incomplete, technically) can be unlocked by clearing True Mercy route alone.
There is no need - for now all but one route-specific content locks are reversible. You can t̪͉͔̹̲̫u̱͔r͉͜n̡̝ a̮̬̣͚͙͍l̳̗l̮̗͕̬͉̲͢ ̦̳̠͚͙̱ṱ̵h̬e̸̝̗̪͍̺ ͍͔̦̭̦d̠͈͈e̥͓̫͉͜ͅv̢̹i̠̣͇̻͙̠l̥͙͉̱͉̤s͏̺̳ ̗̫in҉͚̗͎̬t́o̹̲̥̦̱ ͠a͔̠͍͉̠̟͚ s̖̱͜t̺̗̖̯͔̘̟ea̖̲̖̦m͕̩̮̦į̤͚͕͔͍͍ng̰̝͓͈͇̼͡ ̨͚̰̤͎̱͇p̷͕̱̟i̜͈̺̲l҉̠̳̹̬̦͖̩e ̧o̲̰̯̰͕̟͜f͕̖͜ ͓̮̜̰͍g͇͇̗u͍̩̗̲̯̤̲r̜͕̮o̭̘̠͚ and then just save all of them two runs later like nothing have ever happened.

Do toy know which one i am missing. These are the only 5 ???? i have left.