I've also witnessed and/or been involved with brewing fruit beers in several breweries, and with the exception of orange zest/peel, none of the fruit was added to the boil. So I'm not going to automatically accept that most microbreweries do it that way.
Thinking about this logically, if boiling makes the aroma more "solid," it must be one of two things:
1) It's changing something chemically. But if so, how is that a more authentic fruit aroma?
-or-
2) Greater extraction. But I find that had to fathom, given that I've tasted and smelled spent fruit after secondary, and it was pretty devoid of flavor/aroma.
Regarding the breweries that do and don't do it that is all about anecdotal exposure so that is a fair enough point, I was just expressing that they used to only do it in secondary (I should have made that more clear) and their fruit beers didn't do well. When they started doing it in the boil too, fresh fruit, most of the local breweries (where I am there are 13 or so) started seeing better results.
Looking into why has more to do with your first inquiry. Chemical in a culinary sense. As far as my training and experience is on the subject authenticity is not what anyone is looking for in the direct sense. Adding the fruit in the boil adds more complexity to the fruit flavors and smells, even adding the nutrients of those fruits for the yeast to enjoy. It's not that it necessarily makes it more authentic, I dont find that as a way to define flavors, aromas, and tastes because some people, normally hipsters and peculiar purists, would argue that same thing. (Like you can't call your yeast champagne ywst because it was not cultured in that part of France..) those people can die in a fire for all I care.
To get back on point, adding in boil and in secondary makes the fruit notes more complex then just in secondary alone. Now, this varies from brewer to brewer and how much fruit you're willing to use as the more fruit the more it adds. But I would say that the majority of fruit in secondary and a half pound or a pound in boil is more than enough.
The stewing of fruit, usually dark fruit in dark beer, seems to add weight to the fruit notes as opposed to only lightly feeling the weight of fruit at the end. I do a Black berry stout once a year and tried it out myself. Upon adding to the boil, my frie.... lowlifes who drank the stout noticed that the fruit notes followed them all the way through the tasting of the beer. It smelled of black berry amd it stuck with them as they drank it. Not sweet or hyper fruit strong, it was just something they immediately pointed out to me as different when they tried it.
From that point on I looked further into it and started doing it with all my fruit beers that I do in summer. There was a difference... quantifying it is hard because people have different tastes and sensibilities to said tastes.
So, in summary, it seemed to keep the fruit notes throughout the tasting of the beer better, the aroma was more complexed making the fruit more distinct when on contrast to the hops (not outdoing the hops by any stretch of the imagination, but it would be better defined or seem better defined when compared as some hops can over power fruit flavors).
A neat culinary example is commercial orange juice. Most of them are bogus and they use orange flavored sweeteners and fake chemicals to give off the smell of oranges and your brain, in that moment, says "yum, orange juice". Real orange juice has a very different taste to it when you directly compare the two. Now some brands of oj are mixing a lot more fruit back into the mix (I believe during pasteurization) because it does a better job fooling the consumer that it is natural on when it is mostly a mixture of fake and real stuff.
This is just an example, I would not fully compare this to brewing but case in point, when fruit is in secondary only you miss out on the long term wart getting to know the fruit in the beer overall during the full fermentation. Nutrients from the fruit, flavors, aromas that come from the last bit of the boil due impart structure to the fruit profile in the beer. Secondary is absolutely the most important part of implementing fruit to a beer and if you're on a tight budget I'dsay only do it in secondary, but the complexity gained from the boil helps it out just enough that it, in my opinion, is worth doing.
The only thing I couldn't tell you is how much fruit to use with regards to different fruit amounts on how potent they are. If I have not used a fruit, let's say kiwis as I have not used it in a beer yet, I'd be reluctant to advise how much fruit to add. I've not directly used guava, but have heard from others it has a distinctive element to it's taste, so I would caution myself on how much I use. I have never even eaten guava, so now I am heading to the store to get one and try it sometime tomorrow. I've had it in beer, but not the actual fruit itself now that I think about it. XD
(I am out at work atm so I'm sorry if my replies are a little late)
