Three questions i just got about my dog’s breed’s behaviours

So, my 8 month chiweenie (Mix between chihuahua and Dachshund) has done pretty weird and sometimes funny things and idk how to correct them

1. He gets onto the table when we leave him alone, this is kinda scary and worries me because i always get the question, What if he eats something he shouldn't?, He has ate onions before and i mean a bunch but, it still scares me

2. At some point of the day (usually at night) he will start running through the house (my house is a small one so he runs from my room to the other room over and over, taking some stops to drink water), i wonder if this is because he's stressed or something (tho i take him out)

3. His heartbeat rate increased when we went on a car trip, he was also panting, was he overwhelmed because of the small space? (Note: It was his first time on a trip)