Is using a ladle a kli sheni?
It would depend.
If the pot is off the fire and the ladle is immediately withdrawn with the liquid than yes it can be considered a kli sheni.
But if the pot is on the fire OR the ladle is inserted into the pot until the ladle it self is hot even if the pot is no longer on the fire then it would not be a kli sheni.
Even if the pot is on a hot plate with a blech, like aluminum foil on a crockpot base?
The blech has no bearing on kli rishon or sheni. A pot on a blech is still a kli rishon.