1) As far as I remember, the only major thing blocked by main quest is spell shop for your character and branding of captured slaves (I am not really sure about this one, as I never really cared about quests). I guess if you want to be a mage, then yes - you are fucked. But you can still fight, unlock abilities for warrior-like character, lvl up, travel, etc. I don't really know what more do you need.
2) You get 250 at start. Leave mansion, go to the slaver's guild and buy someone in range of 50-150 (>120 only if they are actually good). Go home and click on their names. You can choose their occupation with the middle-left button and you can see slave's stats/mood on the top. You can go to main mansion screen (Q) and click on interact button for mood stabilization.
3) On the stat list mentioned before you can see HP bar, Stamina bar and EXP bar. When it fills, you can talk to them for a lvl up. After day of work you can see that the meter fills up.
4) I don't really get this one. Buy ropes from market, exit city and hunt for more slaves. More slaves = more workers = more money.
And just because you can't play the story, because on your own self-imposed rules, doesn't mean it should not exist. Set ration lower, get the girl, give it away and put ratio back to 100:0.