BAH, a day hike in to T.I. Lake and back is a full day. And you can't fully appreciate the lake unless you walk at least half a mile along its shore to see the islands and its beauty.
I would think you will have patches of snow, where the trail is in shady trees. But that is only a guess. It shouldn't be too much trouble, though, since snow that late is well consolidated, so you should be able to walk over it.
Definitely take the PCT (higher) trail, as it is up out of the trees. I would call the ranger station in Mammoth and ask about the road conditions before you go. As Rogue said, it might not even be open.
Just for fun here's a report for you:
Kayaks on Thousand Island Lake
