For division cards, how do you know when the abilities can be used on other players?
For example, I assume you can use the barracks "treat unit as an instant card" ability to place a card either in your own lane or someone else's. But I also assume only the player with the science lab division can get the +2 damage when using technology cards (that is, even if a different play plays a technology card on the player with the science lab division, the +2 is not applied since the card did not come from the science lab players hand). For weapons research division wave +1 ability, does everyone get this discount?
Do pretty much all abilities stack? For example, if you have two of the shield slugs in lane, then defense is increased by 2? Or if you had to fight two of the drones that make you draw 1 less card, then you draw 2 less cards? (assuming each killed an ally)
What is the point of boss cards? Seems sometimes you wont even face them. Is it just to make it more difficult for 3/4 player games?
Can you assume any "+/- power/health traits for all enemies/allies in lane" abilities on a card also include the card itself? For example, the medic can prevent damage on itself? Or another example, shield slugs give themselves +1 defense?
Can rangers/militia essentially serve as xenosathem? That is, could you burn two militia to buy a single ranger?
I will try to answer your questions to the best of my abilities.
Division cards - Basically you have to use common sense. If it is somewhat vague, usually other people can use the ability. It also helps to search the forums, as they have most or all of the answers to that.
Yes, abilities generally stack. Two "shield slugs" in lane stack. If you fight two drones who each kill a troop, it means you would draw two less cards.
Boss cards usage have been changed with the expansion. The designer now recommends having 3 hive cards in the first round of a wave, followed by 4 hive cards in the second, followed by 3 hive cards and 1 boss card for the final round in a wave. Search for the dreadmire updates to the base game in the forums. Otherwise, using the base rules, you may face a boss with 1-2 players, or up to 2 bosses with 3-4 players. Sometimes you won't face them but you never know when they will show up. I think that they just add additional challenge and replay-ability to the game.
Lane abilities of the shield bugs and other include themselves. And yes, medics can prevent damage to themselves.
I'm not a 100 percent sure, but I believe you can use the miltia/ranger cards in the fashion you state. So you can trade in two milita cards for a ranger card. I know that they can stack and add additional discounts to one card. Also note that in the expansion, Dreadmire, the stats have been changed for the Ranger and they also count for a -2 discount instead of -1 like the Miltia.
Hope this helps!!