The answer to your first question is that you pretty much have it right. Str/End and heavy armor type go to khem. If you want him to be a meat shield, defense is also good. If you plan to put him damage mode, crit and alacrity would be decent choices.
Second question is that in your preferences you need to turn on the feature 'show companion tool tips' - I can't remember exactly which category it's under (I can't log on from work /sadface). This will show his stat changes right next to yours when you mouse over a new piece of loot.
I'm pretty sure Khem Val has no use for Alacrity.
To the OP, once you get past level 30, tanking stats (defense, shield, absorption) become important as well. I'd go Endurance over Strength also as he is a tank.