Congratulations on completing all quests! You are actually the first one to complete them on CityMania, me and cordal did it back when it was called Novapolis.
As for your questions, it's a bit hard to understand your english, but I think I got most of it.
I think of adding option to redo quests but it probably won't happen anytime soon as there aren't currently many players who would've used it. So for now you can create second account if you want, it's perfectly fine.
And you're right, we aren't getting many players on our servers, I'm trying to do something about it but so far nothing really worked. Oldschool players rarely play nowadays and new ones don't seem to grasp what's going on here and just prefer servers that already have a lot of companies.
By delay you mean have game paused for some time before it actually starts? OpenTTD doesn't currently support it. I can set it in a way that it will unpause with first company created, but some players seem to not understand that and just leave. Besides, quest timer starts when you create a company, not when game starts so you can spend some time planning the game while it runs. Unless you need whole 6h for quest ofc. Anyway, I'll think what to do about this.