I just extensively tested this with rookies and the results are pretty weird.
My worst throwers have an accuracy of 51 and they can throw +25 tiles without missing, they hit the target tile with 100% accuracy -
always!
Weaker ones may have a limited range but they never miss, no matter how far I let them throw.
When using heavy- or auto-cannons at this range, the results are terrible even with aimed shots.
It wasn't like this in earlier builds, I think this is a bug.
About alien grenades - if you play at higher difficulties, more aliens carry grenades so you have a constant supply. Never had to manufacture an alien item in the entire playthrough.