Crate time; explosiveness; office dog

Looking for some advice on the following questions! I have a 4mo (huge!) golden retriever. Love him to bits, still lots to learn together.

1. If you go outside the house during crate time (eg. crate time with us is from 9:30-12:00) and return before crate time is finished do you leave the puppy inside the crate upon returning home (and ignore all the excitment)?
2. My pup tends to be very explosive and our dog trainer describes him as such. He can suddenly jump at you so quickly, start biting in the direction of your work eg. Because it’s unexpected behaviour I’m not sure how to deal with it. I live alone and I am very calm so there isn’t anything to overexcite him.
3. My puppy comes to work and generally does very well once he settles in. Upon arriving at the office he get’s so overstimulated he cannot really listen or stay calm, stop jumping. I have my colleagues ignoring him during this time for weeks (turning/ignoring if he jumps, only giving attention when he's calm) but he stays very over excited in the morning and during lunch. really taking the fun out of office dog. Any people who are taking their dog to work who can share some wisdom?