A thrown stone would be a small bash attack and use Thrown OB for rock. In certain circumstance, fall/crush table may be appropriate.
Thrown underwear would be a small grapple attack, small tiny attack if crusty.
OB is by skill, or appropriate stat x3 if no skill.
If you wish to house rule thrown weapons, do so. Your reasoning seems solid to me. I, for example, do not use the Pole Arm category. Pole Arms are TH Weapons and are placed in that category. I do not use Qu penalties for armor because those penalties are built into the attack tables. Go with what you think is right. Make the game yours.