1.) No, when a Licenti separates a part of itself, with very few exceptions (so few I'm not going to explain otherwise for now) it loses control of the part it separated. If the part is large enough to create a parasitic offspring, then the host may eventually hear the Licentia, but if they do it's probably slowly killing them.

2.) Even if it did hold control, they wouldn't share a consciousness. A Licenti cannot impart knowledge of its language to another.

3.) If you wanted to become the "symbiotic," thing, you could be a Licenti in their Growth stage who has latched onto a human, and they willingly allow it. The Licenti could have telepathy and so the two would work together by communicating telepathically.

4.) A single Licenti tends to modify its structure many times in its life, and quite frequently while it is in one of its Growth Stages. The younger it is, the more the form is likely to vary during a Growth stage. So yes, it could do what you suggest.

5.) Resistance against weaponry depends on what sort of form the Licenti take. Some will be more vulnerable, others will be less.