Can I leave Thailand while my Non-Immigrant B visa extension is under review?

Better to make sure you can do as Tod says with your employer and have them talk with the immigration office.

𝟭) 𝗜 𝗰𝗮𝗻’𝘁 𝗿𝗲𝗺𝗲𝗺𝗯𝗲𝗿 𝗶𝗳 𝘁𝗵𝗲𝘆 𝗵𝗼𝗹𝗱 𝗺𝘆 𝗽𝗮𝘀𝘀𝗽𝗼𝗿𝘁 𝗮𝗳𝘁𝗲𝗿 𝘁𝗵𝗲 𝗮𝗽𝗽𝗹𝗶𝗰𝗮𝘁𝗶𝗼𝗻 𝗮𝗻𝗱 𝗶𝗳 𝘁𝗵𝗲𝘆 𝗱𝗼, 𝗵𝗼𝘄 𝗹𝗼𝗻𝗴?

They do not hold your passport at immigrations. They issue you a 30 day under consideration stamp when you apply for the year extension

𝟮) 𝗔𝗺 𝗜 𝗮𝗯𝗹𝗲 𝘁𝗼 𝗹𝗲𝗮𝘃𝗲 𝘁𝗵𝗲 𝗰𝗼𝘂𝗻𝘁𝗿𝘆 𝘄𝗵𝗶𝗹𝘀𝘁 𝘁𝗵𝗮𝘁’𝘀 𝘂𝗻𝗱𝗲𝗿 𝗿𝗲𝘃𝗶𝗲𝘄?

Yes, you can leave the country IF you buy a re-entry permit (which will be valid ONLY for that 30 day under consideration stamp) <- that means you have to return before that stamp runs out so you can go to the immigration office on that date and get the year extension inked in.