Is this the place to park and hike the backside of Flat Top? I have only gone up the front side. Are the trails clearly marked so we know which one to take up? Thanks in advance.

Yes, the parking lot is not as large or formal as the "front side", but there are plenty of places to park in the fee area. The Sunnyside trail is clearly marked and if you go a bit further down the road you will see the Rabbit Lake trailhead. Sunnyside starts with switchbacks and then a smooth sloping climb to the top. None of the scrambling that you find on the front side of Flat Top.

Yes. The trail for Flat Top "Sunnyside" is clearly marked. There's a Porta-Potty there and some new fences on the trail. Can't miss it.