How to spend attribute symbol on Gear card?
Unless it says to discard, no, you may keep using it. 

Yes, you automatically get it if you assign it to the crew on the mission. Each crew can only hold one equipment per mission (with some exceptions for specific crew and equipment). See the main instructions for more on the rules for equipment carrying limits and how mission assignment works.

Assuming you have multiple crew to carry each of the items you're using and they're all involved in the mission at this point, yes, they skill icons stack for skill tests.