I couldn't find answers on the Internet to some of my own questions, so I made some tests. I wanted to share my results. I hope some of them could be useful.
1) Enfeeble is applied to itself
Enfeeble extends the duration of hostile effects. And enfeeble is an hostile effect iself.
Enfeeble does extend its own duration. That means, for example, that Heart Seeker applies 30s of Enfeeble on non-resistant foes (without taking into account INT and PL).
Additional tip : that al