Yes, you can hire mercs again if you dismiss them. The MC's willpower can be raised through the persuasion skillbooks I believe, as well as teaching at the academy and some random events. Balancing skill training is an ongoing process, and the next update should help a little in that department, by increasing the chance of success if the slave's discipline is above 30. Also, if you don't have the stats to effectively train a slave's skill yourself, it's much more effective to have her attend the relevant course at the academy, which will passively raise her skill over time. The tradeoff being that she'll be out of the house for class for a few hours every day, but this does mean that you can go out and do stuff without worrying about her during that time.
Respect is very difficult to raise, with taking the slave into combat being the only consistent method i've found, but it's honestly not that important of a stat. If your slave's willpower is low and discipline is high, everything else just sort of falls into place. Ranged skill is entirely independent of dexterity, dex is used for dodging melee attacks and such. What's your ranged skill at? 75 is considered above average, and once you hit 80-90 or so you'll likely see a significant slowdown in skill gain, and getting above 100 is very hard indeed as by that point you'll almost never miss. You will always gain one skill point from landing a critical hit however, which has a flat 5% chance of happening, so just using your ranged weapon in combat will naturally see you raise your skill over time.
