Well, I'm an eternal optimist, although I have for ages experienced loss of a lot of content (sometimes even from RL customers, when a wrong click on a checkbox suddenly makes 3000 prims disappear in a microsecond, and there is nothing — short of yammering at LL — that makes them come back).
So, yes, setting permissions [i:1kzk3yf3]right[/i:1kzk3yf3] is an utter nightmare. The worst case scenario is exactly what we have in Neufreistadt (Colonia Nova, having been planned from scratch with a building group in mind, fares a bit better): lots of builders, each on their own group, each setting permissions to their own taste, each trusting that they will always be around to fix things (but of course leaving SL few months after claiming otherwise...), and setting prims to piles upon piles of groups with wrongly defined roles, even when sometimes the builder of the object is the same person!
There is no magic button to change [i:1kzk3yf3]group[/i:1kzk3yf3] ownership and "fix permissions" to the right group/settings. LL's employees are very reluctant to do that, since it means transferring ownership of content without really asking the owners if they agree. The [i:1kzk3yf3]best[/i:1kzk3yf3] that LL provides is a transfer of [i:1kzk3yf3]personal[/i:1kzk3yf3] ownership [i:1kzk3yf3]without[/i:1kzk3yf3] touching the permissions or the group settings: this is done by having a plot reclaimed by the Estate Manager and then bought again — with all content changing ownership. This allows to "fix" multiple ownership, but it doesn't fix group membership, which has to be done manually, prim by prim.
So the "best" way so far to deal with this is the following way:
1) Have separate groups for "building" and "visiting" (no matter how good the new group tools are, there is no way to create a group role that allows to deal with both situations).
2) Change the prim allowance bonus on the region to 10x (better safe than sorry!)
3) Re-claim a parcel of public land with all content. Then re-buy it for a single individual [i:1kzk3yf3]with all content[/i:1kzk3yf3] (that's an option when setting the land for sale). This allows all content on that parcel be assigned to a single user, and this facilitates some changes later. Permissions are not affected.
4) Set the content to the same group (ie. the building group). Share it to group ([b:1kzk3yf3]not[/b:1kzk3yf3] "deed"!). Open up permissions as much as possible (in some cases, this will be impossible).
5) Repeat for all parcels with public content on display.
6) Re-check that [i:1kzk3yf3]all[/i:1kzk3yf3] parcels on public land are set to the [i:1kzk3yf3]same[/i:1kzk3yf3] group (this is [b:1kzk3yf3]vital![/b:1kzk3yf3]). To make double-sure, you can use the group tools to see if all parcels show on that group. This sometimes is not instantaneous due to the complex caching mechanisms in SL, so it's best to try a few times to make sure that no parcel is "missing".
7) Count prims. The most tedious work overall! Basically, what one needs to do is to make sure that all land with public buildings, added together, has enough prim allowance to hold all public buildings, even if on individual parcels there is some overflowing (eg. the church parcel).
Turn down the prim allowance bonus back to 1x.
9) Pray, and have a Linden standing by to apply the rollback in case something goes wrong 
As you can see, this is a daunting task, and once that has, at best, 70% of chance of success, at least in my personal case, who just did this half a dozen of times.
The end result is a bit "dangerous": it means that everybody in the building group will, to a degree, have some access to the public buildings. While this makes sense during a building stage, it doesn't make sense on a subsequent stage when the buildings are done. So ideally, everybody that does not [i:1kzk3yf3]need[/i:1kzk3yf3] to be in the building group should be thrown out of it, and only invited on a case-by-case basis as needed.
The rest of the roles on that group should be as Samantha so thoroughly defines.
And I agree on two points there: the first is the "Ulrika trauma" (ie. people unwilling to set public content to group and have it copied or broken apart), and a second one is the mess of setting groups properly which takes a huge amount of time without [i:1kzk3yf3]guaranteed[/i:1kzk3yf3] success.
The first issue [i:1kzk3yf3]can[/i:1kzk3yf3] be dealt with by [i:1kzk3yf3]paying[/i:1kzk3yf3] people for developed content. For Colonia Nova this was better agreed up front (still not dealt with; see the discussions on other threads): that builders were [i:1kzk3yf3]expected[/i:1kzk3yf3] to turn over their content to the Government [i:1kzk3yf3]after[/i:1kzk3yf3] the building was done, and get payment for that (or, when working [i:1kzk3yf3]pro bono[/i:1kzk3yf3], send the Government an email granting it the full use of their created content). Of course, the issue in Neufreistadt is way more complex. BladeDragon, for instance, who is a pretty nice guy, has built the Church before even there was an elected government. We're just lucky that he happens to log in to SL a few days every year, and he's always happy to see that we're still around after so many years, and so is his Church Kendra, unlike what might have been suggested in this thread, will actually be very glad to change whatever we ask her — she holds no real grudge against us, it's just a question of asking gently, paying our dues when that is an issue (in some cases), and arrange with her a date and hour to be around. Older content, from people that aren't in SL any more, is way harder — in some cases, it would make more sense simply to delete & replace that content, either through a public contest or asking the New Guild for help.
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. IMHO it's time to put that phobia behind us. 
I sumit that the Marketplaz is a good place to start because it will teach us about group tools.
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