"does everybody in your party gain experience from battles, or only the ones who participated in (and are alive at the end of)the mission?"
no- only the ones who participate in the mission- here is how the system works- after each turn for each character (if they did some action such as fight or tech or magic or item use) that character gains a certain amount of experience points which are then shown on the screen- such as "gained 12 exp."- not all characters get those points, only the one who did the action- once you reach 100 experience points, that character levels up, and thus takes another 100 to level up agian- so you must do actions to gain experience- if you just move but do not take any actions except for wait, then you do not gain any experience points
" understand that you earn magic abilities through your weapons/armor. and after you get enough experience points you gain the magic abilities even after you equip other weapons/armor. but what happens if you're halfway through mastering one ability and then you switch to a new weapon/armor, and then you later switch back so you can master the ability? do you have to start over at mastering it, or do you continue where you left off?"
as far as i know the ability points stay with you so if you take the equipment off half way through, you can put it on later and retain the ap from that original equipment- when i had my magic rod on my one character i had 3 magics, but i didnt let it gain enough ap, and i switched to another weapon- i got the magic from that one, then switched back and i think it kept my ap in the other one since it only took 1 battle for me to master the 3 magics
"is there any way i'm supposed to know where you place things on the over-world map? i'm told that if you place things in a certain order you get to go on treasure hunts and earn rare items. is there any way i'm supposed to know where to place things, or is it all blind luck? is there any way i can move things once i place them?"
i think you put the places on the world map by yourself where ever you please- and to get into the treasure hunts is random i think- it will randomly appear on the menu selection for that town or place- you then choose it and violla instant item!
"what's up with the days on the over-world map? it takes one day to travel from one spot to the next. is there a time limit or something? do i have to complete goals in a certain time frame or face the consequences?"
no- it is there for some dispatch missions- in those you send out a member of your clan and sometimes must wait a certain number of days until the mission is finished- some take 5 days, others 10- some dispatch missions also require other things such as you must fight 2 battles until it is completed
"lastly, are there any missions/events that i can only complete at certain times and if i miss them i don't get to witness them unless i start over?"
i dont know- i havent gotten too far in the game just yet, but for right now i have yet to see any
" lied, one more; does anyone know if there are multiple endings to the game?"
i dont know the definate answer to that, but i would say yes- and it is probably determined on the number of missions you complete- there are 300 in the game- completing all probably would get you something
well i hope this helps

if you have any other questions just ask