Well I have totally no idea how to work this thing.
Plus I'm almost done with the game. At least I hope so.
Not sure how I missed this. This is more easily done on a new character during the creation sequence, but here goes.
When you have CheatEngine running, hit the button on the left to open a process, select the process for Underrail. Go to your Skills screen and it should show 120 skill points available (40 on later level ups) and type 120 (40) into the Value search bar just beneath the scan buttons. Once you see all the results, go back to the game, use one skill point, then go back to the search bar and type in 119 (39) and hit Next Scan. You should have just one address pop up at that point on the left. Right click that, select Add selected Address to the address list. The string of numbers will then be added to the bottom. Click the box to make it active. You now have unlimited skill points to distribute. Uncheck the box after you max out your skills (for the level) and right click the address string at the bottom, scroll up to Change Record, then select Value. Hit backspace until there are no numbers below the Value column and hit 0 and then enter.
For stat points at creation, then you hit New Scan, then type 5 into the search field, hit First Scan, spend one stat point, go back, type 4 into the search, Next Scan, then back in the game, spend one more stat point, back to the search engine and type 3 in for Next Scan and you'll be given the address. Right click that address, add it to the address list, make sure the box is checked, and you now have unlimited stat points. Spend them, go back to Cheat Engine, right click, change record, set the value to 0 like before, hit enter, uncheck the box.
At this point you'll notice that aside from the last two digits in the address, the stat points and skill points addresses are identical. During character creation, you have extra feat points as well. Repeat the steps mentioned for skill points, only use 2 as your first scan. Take one feat, then go to the search and type in 1. Since you should know what most of the address looks like, finding the one for feats should be easy. Now, when wanting unlimited feat points, once it's added and made active, change the value to 4 and hit enter, so you have enough to spend. Grab what feats you want (increase your skills and stats before since feats have requirements and availability will update even during the creation/level-up screen).
Now, on later level ups, you might only get 1 feat point, at that point you make sure you edited your skills first so you know what most of the address will look like, then search for 1, grab a feat, type in 0, Next Scan again, put that feat back, search for 1; eventually you'll find the address you need to edit. Treat it the same as you did during character creation.
Eventually you'll be able to max your stats (level 4 using the same trick to search for stats). At level 8 however, you'll be given a stat point with nothing to spend it on, so you can't increase or decrease it to narrow down a search, namely through looking at the address for skill points left. Right now during character creation (started it up to do New Game to walk myself and you through this), my Skill Points address is 04334940 and my stat points address is 04334944, that means that all you have to do is add 4 to the last digit for Skill Points to be able to locate the one for stat points (if the last digit is 7 for skills then the last digit for Stat Points will be A, add 4 to 8 and it's B, etc). Then you add the address for stats, right click to change the record, type in 0, hit enter. You won't be allowed to add stats when all the stats are maxed, but you still need your available stat points, skill points, and feat points to be 0 before you can move past the creation/level-up screen.
Does that help you? You can always make a brand new character and at the screen to make them, you can go through the steps mentioned above to get the hang of it.