Can all all ability bonuses aside from weapons "+" effects be stacked?
1) +N bonuses do not compound. From rules (Section Additional Rules - Tests p.12:


  An investigator can use only one card effect that provides a skill
  bonus during each test (for example, an Asset that reads “Gain
  +1 [”). If he has multiple card effects that provide a bonus, he
  uses the highest bonus.


An ally is an asset just like a weapon, so having +1 to Strength from an ally and a +1 to Strength from a gun will result you in having a +1.

2) Yes. From reference guide (Section Reroll): 


  If multiple effects allow an investigator to reroll a die, he may
  resolve each effect separately. There is no limit to the number of
  times a die or test can be rerolled, so long as the investigator has
  the means to do so.