So, I played around with the dungeon and it seems to scare slaves into postive obediance levels. They are unhappy but will do what you ask most of the time. Casting magic so you can give them rewards that balance out the despair feels odd to me but I guess I could just let them go mentally. If the dungeon had stocks it would make sense that you could train a braindead sex slave down there but being that you bring them up to the regular slave slot there is nothing really keeping them from attacking you or killing themselves. I think I'm far enough along that they simply won't do that anymore but I don't know. I have all of the quest reward items equiped and I noticed you can pretty much encourage a positive attitude indefinetly. Doing that allows you to keep them happy and motivate them to train and then reward them to build their stats. Only draw back is they don't sleep well in the dungeon so energy levels restrict how much training can be done. Some characters are not bothered by being left in a single place and not moving around. I would say they are the best for this sort of thing because they don't get an attitude problem from sleeping in the dungeon.
So, I decided to try out training two slaves at once and let each slave sleep in the dungeon every other night. I would say it's profitable but there are many drawbacks to this. For one, characters being trained need to be fed much more than normal and so does the trainer. Multiple characters need multiple trainers and so it results in double the food costs which are much higher than rent so it gets pretty costly. The other issue is it takes longer to train them in this way because things don't always work out and they have less energy to train with if they were in the dungeon. It also takes much longer in the real world to complete a single in game day because you have a lot of things to deal with. If you want to maintain your cow while doing this then you will need to pay 20 sparks a decade for the cryo cells as well.
In the end I trained two girls to S+ class at the same time and one of them got done a little earlier than the other because of her starting stats. It took about 160 days and burned through 3700 sparks. Normally if I train a single slave it takes about 90 to 120 days and it costs about 1000 sparks. But you get 2X the payout when you sell them and the reward for each slave far outstrips the rent and food costs. Selling them for about 20,000 leaves me with over 16000 sparks profit. Training individually, I think I could do similar so it's possible but I would say cost vs profit is very similar. Where you gain from this is that it takes you less game time to bring in the same profit with similar training costs. Time is money so it's worth it if you want to deal with all of the added pain of training two at a time. I would say it would have taken 180 to 240 days if I had done this individually but I got it done in 160 days by training them at the same time. However, this took twice as long in the real world to train them and complicated things alot.
Of course, I could just let them go mind broken and pay no attention to the dungeon slave in order to sell it off for cheap with the sub purchaser, but I could also catch girls in the fog and train them as pigs, but I usually don't bother with that either. Perhaps if I were playing on a harder difficulty setting, I would be struggling for cash much more and would need to do these things to survive. If I were doing a speed run or my monthly costs were outstripping my cash reserves then this could be used to bring in profit faster but the farm pretty much covers my costs and my bank account looks similar there are just less days counted at the top of the screen which isn't worth wasting more real world time perusing unless you are under major financial stress which I am not.
I did find a small oversight that I figured I should report. I'm playing by my own rules, so I decided to do all 3 jobs from the nobles in the white chapel of the bull district. I built the horse woman first and then the child tutor for a noble's son and in the description for the job it says the child needs to be clever which is an "A" intelligence, but it's colored like an "S" class attribute would normally be. He accepted a light blue "A" class or "clever" intelligence, but the coloring of the word "clever" in the job description was wrong and confusing.