Moving down without tanking

1. I won a Jesse during the event, so it was level 1 like yours. I just put in 4 lvl 9 Aphids and ran it stock. So level your Aphids up first. Also in Expert iOS.
2. I don't know, may go down into Diamond again. You could stay and play well with that line-up too, so maybe Expert. If you swap your line ups and back and forth to heavies, then more likely to stay.
3. Not long. A lot of people bob up and down between tiers so it's perfectly normal.
4. With the change in MM, it's a lot easier to go back up to where you belong. I was stuck in lower tiers and couldn't get out of it due to bad teammates. I was messing around with different builds, unleveled or underleveled, etc. Since the new moon, I shot back up to Expert despite losing due weak teammates, tankers, etc. so it wasn't my fault and nothing I could have done.

I would say there is a bigger wall between Expert and Diamond since you get lumped with everything above. So when I got back into Expert, I had a team of Champion and Masters and so it will be hard to compete in damage when you get those types of games. That's if your team loses, so it's out of your hands. Also if you go up against a Champion squad, and you team is filled with Experts. Then it's a race to get damage so you try to minimize your loss.

Play the light bots using the weaponry that you would otherwise use for your heavies. Don't duplicate.

e.g. Keep upgrading the two thunders off your thunder carnage - but put them on two thunder shultze.
A Plasma db griffin arms two plasma gareths.

Be warned that the grind down may be slow if you are still playing to win. It may be 'very' frustrating.