I may not understand your questions completely, so I'll see if I can answer them.
If you have the GTA IV full game trial, you should be able to instantly tell if it works with your retail save based on whether it starts from the beginning or not. Nobody has posted yet as to whether they're compatible or not.
Episodes From Liberty City is a standalone game that lets you just play the two pieces of DLC with none of the content from GTAIV, so I don't believe it's compatible with GTA IV saves in any way. If you grab the
Ultimate Edition, you just gets the add-ons for GTAIV along with the main game, so the save compatibility should work the same as GTA IV.