You can buy it here: https://dlsoft.dmm.co.jp/detail/navel_0001/
But I strongly recommend also getting this for fully voiced protagonist: https://dlsoft.dmm.co.jp/detail/navel_0003/
I have done it few times in the past with Cheat engine and HAT but it really depends on the game. I never used it on Amagami but I van't remember if I couldn't get it to work or just didn't even bother trying because Amagami is just easy to read and has very little actual text in it.
LMFAOOOOOO holy shit HAHAHA TOTAL bruh MOMENT om g lol haha my sides bro im experiencing heart palpitations this joke man dude this is EVERYTHING im lol my brain is deteriorating HAHAHA LOL
Download editions pretty often have small modifications because they usually use some kind of DRM when launching the game for the first time causing the hooks to stop working.
Did you do all bad ends, normal ends, good ends for each heroine.
I also don't know what you mean by Extra Story but if it means Murasaki route then I think it was not translated.
I think it will please you that in Riddle Joker they actually addressed this issue. On left we can see the sprite from trial and on the right we have the final version.
As there are some games where heroines are literally kindergartners, it is safe to say that the Japanese heroines must be over 18 in high school, or even in middle school, because they graduate from kindergarten after they are 18.