I don't have UE but just go to where the save games for each version are kept which are probably under "My Documents\My Games\GalCiv2" for Dread Lords, "My Documents\My Games\GC2DarkAvatar" for Dark Avatar and "My Documents\My Games\GC2TwilightArnor" for Twilight of the Arnor.

In all three cases the primary file that sets up and saves your current game configuration is the Prefs.ini file. This file contains al of your latest selections as to galaxy size, frequency of planet types, victory conditions, number of opponents, your opponent races and difficulty levels and everything about the game except for how the races are configured in terms of how their ability points are spent, their political party, etc. For Dread Lords there are a set of *.raceconfig files that specify your own race and your opponent race configurations. In the case of Dark Avatar and Twilight there are a set of *.customracexml files that perform the same function.

So if you want to reset *everything* then simply delete the Prefs.ini file along with all the *.customracexml or *.raceconfig under a specific directory and then when you next play that game new files will be generated that at least start with the default "factory" settings.

There are other differences between the Dread Lords *.raceconfig files and DA/TA *customracexml files. Specifically the raceconfig files in DL get generated for your *own* race only. However if you leave these files in place and later come along and play the next game as a different race and select a race for which a *.raceconfig file exists as an opponent then the game will in fact use that *.raceconfig file to define your opponent. In essence you select what you feel to be the best configuration for your own race and the game will in fact use that same configuration against you in a later game. For DA/TA you have a lot more control from inside the Useer Interface into how the *.customracexml files get generated and used however that's a different and much more extensive topic.

I personally don't like to delete a configuration that I've worked on and gotten to the point that I like to play. Instead what I do is actually save the Prefs.ini and the *.customracexml file to a different directory and then I have multiple directories each with a different starting setup that I go back and forth between. Anyway by just moving the files not only can you get back to a "default" configuration but by moving your customized files back to the directory they came from you can always go back to the identical setup that you played before. 

This is less important in Dread Lords than in Dark Avatar where there is a lot of opportunity for customization of your opponents. I can be a lot of work to do this and get them just they way you want them and it's a good idea to have a "backup" of your preferred configuration just in case you mess it up.

I've never been a fan of campaigns in general.

As far as which game I might not be the best person to ask. With a limited amount of time to dedicate perhaps the best to go for is right to Twilight. It's a lot more complicated with separate Tech trees for each race but to the best of my knowledge Twilight has the fewest bugs and has reasonable gameplay. My problem is that I really haven't played Twilight myself and so my knowledge of it is limited.

Between Dread Lords and Dark Avatar I would probably have to go with Dread Lords. There was a huge nerfage of ship speed done in Dark Avatar based on the mistaken assumption that ship speed was being exploited to overrun the AI making the game easier than it should have been. And although to some extent ship speed can be used to that effect it's by no means the only way that a human can roll over the AI and the speed nerfage in Dark Avatar really just boils down to an annoyance that is substantially greater to those that prefer larger galaxy sizes.

But Dark Avatar has it's redeeming features as well. Certainly as far as "the big game" goes you get far more planets in a DA game versus a DL game.

All in all I really don't think you can make a bad choice as any version of the game that you choose to play is both immersive and highly addictive.

Actually, in my experience, TOA has the most bugs by far.

Just out of curiosity, and only tangentially related to the thread - do you know why starting funds in TOA were reduced to 3000 bcs (instead of 5000 in the other two games)? It's a major annoyance that's almost harder to adjust my playing style to then the unique tech trees (except for the radically altered ones such as the Thalan and Yor) and discourages me from playing TOA in much the same way you seem to dislike the DA 'speed nerfage'. Was it added for any similiar reason?