SureAI

I was wondering : if I install the Steam version of Enderal, will my current old version be compromised?

In other words, is it possible to have both the old version and the Steam version installed and playing both separately?

Yes, that is possible. Enderal Forgotten Stories gets its own directory and its own savegame folder.
But an old savegame is not compatible with FS.

OK, thanks, I'm going to do that, then. Although it's less important now because I finished the game much earlier than I expected (I tended to sidequest a lot in between two main story quests, but once the city was under siege, it seemed to me it was making no sense to roam around the countryside, and I headed straight for the main story, and realized I was closer to the end of the game than I thought) so I don't really need to keep my old saves anymore.

[+] Spoiler

However, I'm now not sure I'll be able to play it again. My problem is that now, knowing the endgame, I'm not motivated at all to do quests and such because everybody will be soon dead, and so what I do doesn't matter, in fact. I could completely ignore the main quest, but then the game would seem pointless. And anyway, I still would know that this world is doomed, even if I'm not involved (I'm sure that if the chosen prophet fails to behave as expected, the high ones will produce another). Not even counting the fact that I know I'm a ghost. I tried (with the old version of the game) and couldn't feel involved, thinking about how they'll die in the cleansing soon when interacting with NPCs. The setting and mandatory gloom ending makes really difficult for me to play the game again.

That is something we all faced, when playing and replaying Enderal. Choice is yours, in the end. Though perhaps the ending will not seem as difficult to swallow, if you have played through the whole story and saw how and why things ended the way they did.