I experimented with this a bit, here's what I found:
- Henchmen can be hired out any any level (with a max level of your current mains +1). You can recruit a level 1 henchman even if you are level 10 with your mains.
- Henchmen will gain XP as long as they are traveling with the party
- Henchmen will NOT gain XP if they are waiting for you in the Hall of Heroes
- Companions (Jahir and Madora) WILL gain XP if they are waiting for you in the Hall of Heroes.
If you want a dedicated character for crafting/blacksmithing, you may want to leave Jahir or Madora at the Hall of Heroes, so they can gain XP which can be dumped into non-combat skills. Replace them with a henchman in your party to fill whatever combat role you need.
Regarding loremaster, it seems that it's easy enough to find gear with +loremaster on it. Level 2 is all I have needed so far (though I am only about 10 levels into the game with my mains - maybe you need more later), and I have been able to get that with gear. Also, it's pretty cheap to have merchants ID items, so you may want to just pay for the service for items that require loremaster higher than 2 rather than invest into the loremaster skill.