We moore our boat for the summer season but use huge permanent concrete mooring anchors, obviously not what your looking for. The Y-anchor concept seems pretty good and ought to work just fine especially if you want to remove it afterward.
As far as moorings moving during a T-storm I've seen it and it ain't pretty. We used to allow folks at our club to manage their own mooring anchors and quite a few of them went with the concrete, rebar and bucket program. Trust me it doesn't work all that well. I've seen several boats pinball their way through the mooring field off other boats on their way down the lake. Ouch, especially when its a party barge! I believe concrete looses its mass in water more so than steel.
Not too sure if you plan on using a mooring ball but if you do I recommend you use a noodle (fat one) from the bottom of the ball to where you plan on attaching it to the boat, then lock it (run the chain through the noodle, may have to cut it then zip tie it together). This will keep the mooring ball or whatever you choose to use from sitting directly under your bow when there's no wind and prevent the grab ring on the mooring ball from scratching the side of your boat when bouncing up and down from boat wakes. Make sense? We also use a secondary line to/from the mooring ball just in case the chain comes disconnected for whatever reason. Seen the results of that too.
Does the lake level drop much? If so if you use a carabeener (sp?) quick connect just beneath the ball and run the chain from the anchor through it you can easily adjust the scope and keep it at an optimum length (we use 6' higher than the lake level) to allow the boat to swing.
Excuse all the information, some of the tricks we have found to be successful over the years.
We'll be down in the ORL area next week as per my e-mail. Looking forward to hooking up and knocking off the rust. It's actually snowing up here today,