Glad to see the opinion of a paying player, having words from both sides helps to see the subject with a broader perspective.
Every game needs to generate an income stream, so I pretty much agree with the current advantages that cash spenders receive, since IMO those advantages aren't game-breaking as tends to happen in other games.
More than that, the current situation of F2P players forced us to plan our gameplay better and organize, which I loved xD yet I still resent not being able to get the only 2 event cards I actually wanted.
Regarding the bosses situation, I pretty much spent the entire event helping as much friends as possible, and I found it quite fun. I tried to land as many full-energy attacks as possible, so the only situation that complicated me was when many bosses spawned at the same time, 'cause it forced me to provide weaker help with 1-energy attacks each. My group raids idea pretty much aimed to provide a more coordinated way for every community's members to provide help to each other as effective as possible, and of course, to (everyone) have a better shot against those 8.7m bosses bug.
And speaking of that, I just don't buy that Nutaku planned the 8.7m guards thing.
The fact that they acknowledged the complains about what happened, plus their announcement of revision and compensation (why would they ever have to "compensate" us for something working normally? they realized something went wrong!), plus the japanese wiki's HP scaling for bosses showing the actual expected HP scaling, makes me think that Nutaku didn't planned that 8.7m guards situation from the beginning, but they likely realized a potential extra income flow in that bug, so they left it like that. In theory.
Either that or the actual solution of the bug wasn't under their technical possibilities. That would be understandable, actually.
Just my humble opinion.