What would be 'cheating' for your game does not define the rules for every Player.
Using Skyrim, the SkyRe overhaul mod (do not use it myself, but have watched it in play) increases damage for Players, as well as opponents. From what I am able to tell, this was done to kill deer and other game at lower levels with a couple of hits rather than having to spam attacks to hunt. Armor is treated the same way; improved for both sides, so no side gains the advantage. And the mod itself is modular; one can select which sub-modules to utilize.
In my game, I give myself unlimited gold. As far as I am able to tell, this does not have any negative effect on the NPC's at all. Both sides appear to remain equaling powerful. I also choose mods that do the same thing; adds to both sides, so the PC does not gain an advantage. I simply get to avoid having to backtrack to towns and stores so often, and avoid the tedium.
Same goes for Skill bonuses. Rather than click and make daggers or some other item, I use commands to add a few pts to Skills, and use a montage or Fade to Black moment to sim the passage of time. Tedious practice and repetition avoided; gameplay for my solo game improved. Same results either way, but one avoids the yawns.
The player is not the game designer. The game was balanced via testing and quality assurance by the developer. When you modify the game in any way, you are cheating. In single player games, the only person who is affected by the cheating is the player. In that case, it is up to the player and his/her conscience to accept the cheat. This is where player point of view comes into play.
I use over 100 Skyrim mods. I'm fully aware that I have custom tailored the game to fit my roleplaying requirements. My mods make the game WAY harder (hell I get debuffs for not eating/sleeping/drinking). However, this is still modifying the game to suit my own needs and is therefore "cheating". I'm just not hurting anybody by making use of such mods.
For example (in Skyrim):
I make my carryweight 9000. Now I can carry a lot more items and therefore make more gold/hour than the average player. I cheated to get a gameplay advantage.
I make my abilities hit harder because I want to make the game more fast-paced rather than a chopsticks fight.
A side-effect of this is making Dragons significantly weaker, so now i have it easier than others so I "cheated" to make the dragon-hunting game easier for me than other players (unintentionally).
I could "fix" this by downloading a dragon-buffing mod like Deadly Dragons.
Repeat Cycle of "fixing".
However, in the end you make the game the way you want it to be and accept the cheats. You do not have to justify yourself to anyone but yourself. You can ultimately make the game a better experience and that's what matters in SP games.
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Another example: MMO mods
Those that use DPS counters are able to min-max more DPS more effectively. They gain an advantage in boss fights. So they are "cheating". They don't hurt anybody as they only seek to maximize their output. When top guilds who use DPS counters liberally get a world first boss kill, nobody says "Well it's because they used Recount and DBM!!". It's just an accepted form of cheating. In this case cheating becomes "cheating", because if players accept this bending of the rules...it's not really what we traditionally call cheating is it?