Ok, there seems to be no cheats in both games.
Is it possible to edit in-games values, so that monster do 0 damage, for example? ...And other such modifiactions to make game easier.

Purpose!..

As sad as this request makes me feel, it is possible, yes. Most of the monster abilities are specified in the opp_monsters.xml file, which must be extracted from the game before you start tampering with it. In there, you can alter health, damage, and resistance values (and more) of all normal monsters. Bosses and more unique enemies have their own, separate .xml files instead, such as draugboss.xml. Using RED Tools, you can unpack the pack0.dzip which contains most of the game files, and thus get a hold of the .xml files you want to change.

This is for TW2, by the way. I have no experience modding TW1.

Simpler than that, would be to increase Geralt's Health, Virgor, Armor and resistances to the maximum value the game can allow!.

Changing monster or NPC stats, could have weird in-game behavior during scripted actions!. Don't know..but it can happen...I guess..(i do not have modding experience with Witcher 1 or 2, nor I have opened up the pack files to see the values.)

True enough. That could be changed in geralt_basic.xml, if the OP is interested. :)