First off, Twinky, welcome. Great to see newbs!
I'll try to answers as best as I can, clarify what Enker has said, and see if we can make it all fit for you.
However, what happens when a FO "activates" an unit with only one specialist using indirect fire (a sergent with UBGL for example) : does the full unit is activated , meaning that actions of the other models of the unit are wasted ? What happens with a full squad with indirect weapons (like pathfinders) : does every model in the squad can fire for each command action from the FO ?
Unfortunately, the book is not clear on this issue. When the Forward Observer (F.O.) activates, it uses Actions to attempt to communicate with the "artillery" unit (in your example, this could be a GL Spec within a squad). If communication is made, the unit that is firing indirectly is, also, activated. By the strictest interpretation of the rules, this would mean that the entire squad to which it belongs is, also, activated. Obviously, this could lead to some serious abuse. For example, I could activate my Dragoon Kaptain, who could, in turn, Give Orders (activate) the Dragoon squad which contains an F.O., who could, in turn, by the effect of Spotting for the GL Spec-equipped, activate said unit (hope that made sense).
I would highly recommend discussing this with those with whom you play, as I could see it being a flashpoint of disagreement in the heat of battle.
That said, I will seek greater clarification from the rest of the FAQ Team.
Another question : it is written page 57 of the UWZ rulebook that because "there are no friendly model within mortar squad command distance who can relay the information to the model firing the mortar", a Forward observer is needed. Does that imply that a non-FO model within command distance could be used as FO specifically for purpose of indirect fire ? I've never found that rule written in the rulebook... Is that a typo or an actual rule ?
This refers to the models that comprise the mortar team. In other words, of the three models within the team, if none may draw LOS to the target, then it may not fire speculatively. If, however, a model with F.O. can draw LOS, then the mortar may attempt speculative fire using the rules as written.
This section does, unfortunately, insinuate that speculative fire is conducted differently. Chalk it up to adding a bit of fluff to a description of how this would work as told in subjective fashion, not in a mechanical intructive.