Will the trial version of FLPro prevent FLlite from working if I don't keep the full paid version?

Question

And conversely will the index made by the lite be usable by the Pro ?

Answer 1

The Lite version and Pro version are completely independent of each other and will continue to work without the other being there (or being uninstalled at a later date).

Neither product uses a permanent index so indexing is not an issue. FLPro has a Cache Database for faster searching of Office/PDF documents but this functionality is not in FLLite.