I don't know what you're talking about here. Could you be more specific?
The reason it is worded like Polymorph is because it is like Polymorph. They both replace your body with another body, but when you take actions with the new body it is still you acting. The main difference is the source (and the fact that you keep "passive" abilities like Regeneration with Magic Jar, and get a separate hit point pool).
I can try, but it'll mostly involve explaining my thought process... which takes a while
Basically, whenever I go to interpret the rules for an ability I take the approach of "The whole is equal to the sum of it's parts".
Thus:
At 6th level, a fiend can attempt exert control over a creature it is possessing.
...followed by an explanation of how he assumes control, and how long it lasts.
This paragraph references the 4th level ability of possessing a creature, so those are the basic rules that apply. Therefore a change mentioned here overrides the rules already set in place:
-For the purposes of affecting the fiend as a target, the fiend does not exist.
-He can take only mental actions
-the fiend becomes "part of the creature". This is a confusing statement. Is it here to imply some significant connection to the creature? Or is it here as fluff to help explain why the fiend cannot be targeted or harmed?
Moving on:
When in control of a victim, the fiend of possession has access to all the creature's abilities, skills, feats, and spell knowledge.
This phrasing of this establishes a precedent of total mental control: Not only is the fiend aware of all the creature's abilities, it can now order the creature to use them as it sees fit.
Mechanically, does it gain those abilities itself? No, it simply has access to them, via the body it is in control of.
It does raise some questions though, such as, "assuming it gains control of the creature's spell knowledge, what happens to it's own?"
The fiend now acts as though it is the creature in all respects, until it loses or relinquishes control.
This further reinforces the previous interpretation. The fiend is in no way imparting himself or his being onto the possessed creature, or supplanting the creature with himself and causing it cease to be. He is simply making use of the creature as a puppet.
This also seems to override the previously mentioned rule of the fiend being allowed to make use of its mental actions, since they are an ability of him, not the creature he is possessing, and he is now acting as that creature in all respects.
Next, after the ability score mechanics, comes this:
It can make use of its own spell-like abilities as well.
Why does this need to be mentioned?
That's right, because we implied earlier he loses access to his own abilities in favor of the creature he is using as a puppet.
This suggests he can use the puppet to channel is spell-like abilities and retain some of his original power.
Finally the description finishes with regards to the fiend keeping the creature's base type, but keeping his own alignment. This is probably the most poorly worded part of the entire class.
It states that the
fiend itself is affected by things that affect the puppet creature's type except in regards to alignment effects, which are dependent on the fiends alignment.
Therefore, if I were to fireball the possessed creature, the fiend is the one taking the HP damage, not the puppet he is in control of. Not only does this completely override the rules of the 4th level ability, in addition to contradicting reasonable interpretations of the 6th level ability up to this point, but it also, in my opinion, it makes it drastically weaker in almost every way.
The only logical conclusion I could come to was that they were referring to the puppet creature being affected, and not the fiend itself. So mechanically, the fiend supplants the puppet's alignment with his own. In following, a possessed creature with a holy weapon would immediately gain some negative levels, not the fiend itself, whereupon any reasonable fiend would have him discard the weapon.
...But anyway, that's just my interpretation of what's going on here...