| AverageJoey |
TLDR at bottom.
My interpretation of the written words:
You would become a diminutive bat using the already published stat block for a bat as a guide. You would then use the guidelines set out in beast shape II to adjust your stats.
From the wording of the feat: "You can take the form of a bat . . .". Not a tiny bat, not a large bat. A bat, which is already defined within the rule set. (although props to slitherrr and his mega bat, made me smile and was educational too). I agree with Tarantula's post earlier on why this is the case. Although transforming into a colossal cheetah and crushing a large portion of the city with a single charge would be sweet it would not be allowed.
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Now for adjusting your stats: While it isn't explicitly stated it seems to me that the intent was to limit the benefits given by the feat to those of the tiny form of beast shape II. I agree with OP that giving a character the sizable combat benefits of a level 4 spell with a permanent duration was probably not the intent. The tiny version however reduces strength, removes hands and thus the use of weapons, gimps your CMB/CMD, and forces you to enter a creature's square to attack it, thus provoking an attack of opportunity which will probably result in you being tripped easily to the ground or grappled and choked out.
From the wording of the feat (emphasis mine): "This ability OTHERWISE functions as beast shape II, and your ability scores change accordingly".
The "otherwise" is important here because as already pointed out, a bat is not a legal form for beast shape II. So as in the first sentence we take on the form of the bat then adjust stats from there which we otherwise could not do.
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Yes it would be considered a use of the shape change ability.
From the wording of the feat (again, emphasis mine): "YOUR powers of transformation have been HONED to the point where you can wholly become a BAT".
Seems like the author is trying to use his words to tell us that the intent of the feat is to improve upon a "power of transformation" we already posses. Seeing as "werebat-kin" is a prerequisite my first instinct was to check the rules for the race and boy was I surprised to find a power of transformation clearly listed there (I wasn't surprised of course, excuse the sarcasm, this post is getting long. I gotta have some fun).
Tying the feat to Change Shape ability gives it concrete rules for duration, uses per day, and labels it as an (Su) ability. so no, it won't work in an anti-magic field where the general feat taken in isolation presumably would. It also answers OP's "super bat" question. Yes you get the usual benefits of the change shape ability (+2 Dex and a choice off the chart).
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OP made an excellent point about the disguise bonus. A cat is "tiny", if I see a cat sized bat flying around I'm going to know something is up. Unless of course I happened to be in the Eastern Mediterranean or South Asia where megabats roam ;) Also note that a mega bat is not the same species as a common bat, nor would a large bat be. And to all the munchkins out there about to say, "yeah, but it doesn't say what species of bat . . ." there are only rules for common bats. If you want to be a mega, super, or collosi-bat take it up with your DM.
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TLDR: Bat Shape is a feat that enhances the Were-bat's shape changing ability. It allows a diminutive form not usually allowed with Beast Shape II while adjusting the stats according to the standard rules for bats and Beast Shape II. Therefore it does cost a use of the Change Shape ability and you do get your +2 Dex and a choice from the list for being in bestial form. Full explanation above.
Cheers folks, I hope all that has been helpful to some.