I was in Slovenia Aug 23 through Sept 17. Weather was great. One rainy day. Comfortable temperatures with highs largely in the 70's.
My planning for the trip started as a 14-day trip through Slovenia, Croatia, and Bosnia & Herzegovina, but I found so much in Slovenia I wanted to see that I changed it to 14 days in Slovenia alone... then 14 days in the western half of Slovenia... then 25 days in the western half of Slovenia. Zero regret for spending so much time there.
I'm not sure what the parking situation for the Cows Ball is. It definitely is walkable from Ribcev Laz to Ukanc. I walked the path completely around the lake one afternoon (Ukanc to Ukanc). There is also a bus that has a few stops along the lake.