We've constructed a rink 6 of the past 7 years. Over those 7 years we've worked out the kinks and now it takes roughly 3 hrs, start to finish for assembly before filling. With two hoses, it takes about 8 hrs to completely fill.
With that, to answer your questions.
1. The yard is not perfectly level, nobody's is, at least that is what we learned first time around. Year one we thought our backyard was perfectly level, (by eye). After construction we came to find we were quite wrong. We wound up having to reinforce the left "long" wall because that entire side of the rink was 2 - 2 1/2 feet deep, the right side long wall was 3 inches deep.
Year 2 we mulled over two choices to correct leveling, excavate and level off the rink area and have one weird ass looking back yard, or level off with top soil. We chose top soil leveling and had several tons of soil dumped, which we hand graded with conventional yard tools and our lawn tractor. The end result wasn't exact but it wound up close enough without ruining the backyard. Since year two, the right side long wall is still 3 inches and the left is 5-6 inches. Our leveling was pretty much split straight down the middle of the length of the rink.
2. The "grass" always grows back. We used construction grade seed after we graded the rink area 6 years ago. It grows dark green fast and thick. However, it's about 90% weeds. It always comes back quickly within about 3-5 weeks after disassembly in late March/early April. With the tarp in place from Oct/Nov through the winter into March the grass underneath will decay due to trapped moisture and freezing/thawing of dead grass and weeds. When you remove the tarp, smell can be pretty pungent and overwhelming from the decayed landscape. It's basically the same smell as horse manure. That goes away in a couple days when it dries out. We spray down the tarp with the hose, tape any holes that may have opened up from use with Gorilla tape. We let it dry out and then fold it up for storage in the shed until next winter.
Our kids are getting older now and like skating in the park on the frozen crick as well. So this might be the last hoo-rah for our backyard rink. The biggest benefit they have at this point is skating at night under the flood lights. Other than that, they like the park better.