The first question I'm not as sure on since I've only upgraded. To be honest though I would just keep paying the fee (which is only a $55 difference between the delta GOLD amex. When you get to the renewal date you get the free companion pass rather than a $99 companion pass. Also there are no MQM boosts built into the gold and there are in the plat if you think you'll spend enough for it.
I upgraded from the gold to the platinum in january and received the bonus even though I shouldn't have technically been eligible. Two days ago I bumped from the Plat to the Reserve and got the sign up bonus again. Your mileage may vary but I would recommend calling the number on the back of your card when you are ready to bump up and ask them specifically if you will get the signup bonus and if they say yes go ahead and do it.