Is it possible to set the XP rewarded for kills to always be the maximum amount?

Check SP, there's something like it in the Editors etc. subsection of IWD2.

No. Not that I know of.

The XP you get will always be calculated depending on your average party levels. There comes a time, specially if you are soloing, when the charecter is considered just too strong and receives no XP per kill.

The ease-of-use mod lets you get more XP than normal. There is another mod which gives you double the XP. The author intended its use on Insane difficulty, though there is nothing stopping you using it otherwise.

Well if anyones interested I worked out how to do it you just set the lowest XP value for each level/monster rating in the moncrate.2da file you want simple as that.

Yeah, you can fiddle the moncrate.2da file as much as you like. You can put ridiculous numbers in there if you want, but then the game becomes pointless. I like the modifications that make sure you at least get 'some' XP for each battle, but you don't want to get too much otherwise you spoil the game by becoming too powerful.