My last words.
Apple's advice may well be good - but I'm not sure that's "Apple's Advice" so much as one employee. And frankly, on here we see reports of advice from Apple Store employees that is inaccurate ("iPhoto is only good for 25k photos, you can't use Photoshop and iPhoto, and others I have seen over the years).
Don't you see that these 15k photos will be in an unsafe environment? You really want to continue with that?
See above where I said
1. Make a Back up.
That means what ever you do you can still return to the current status quo. As long as you have a back up of the current - mucked-up - library, it can't get worse.
So the worst that can happen is nothing changes. The Best that can happen is it gets fixed.
Library Manager is not a plug-in and does not complicate how iPhoto works. It simplifies working with multiple libraries for a start.
And remember what exactly it does:
Download iPhoto Library Manager and use its rebuild function. (In Library Manager it's the FIle -> Rebuild command)
This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.
Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.
So, make a back up - that's one level of protection, and then Library Manager is non-destructive so that's double the protection.
Finally: search the forum. It is no exaggeration to say that Library Manager is recommended on here 50+ times every week for these kinds of problems. See how many folks find themselves worse off after using it.
After that:
The one thing you really cannot do is leave the photos in the damaged library. It is really, really unwise. If you don't want to use Library manager, then create a new Library
Recover the Masters from the damaged one
Import them to the new Library.
To create a new Library:
Hold down the option (or alt) key key and launch iPhoto. From the resulting menu select 'Create Library'