I'm sorry but having one more CB wouldn't have kept us from getting thrashed 7-0 by Bayern. What difference would've even Thiago Silva made in those situations? At most he possibly could've kept the aggregate score at 6-0 instead of 7.
Individual players and signings won't make a difference except in how highly the team is rated. We didn't just decline because of bad player signings (although that was an aspect), we also declined because of player aging and major systemic issues. And while you feel you could've made the better choice of going for a CB instead of Song, we still wouldn't have had a much different season. It's not just about the players, it's about how you use them. And frankly that job is much more difficult so it's not as if you sitting behind a computer screen could do it any better than Rosell or Tito or Zubi. Buying better players doesn't automatically translate to more success. Real Madrid thought they'd win La Decima by now by signing guys like Alonso, CR, etc. and getting Mourinho as manager. But it's not that simple is it? The team dynamic and tactical system is what isn't working for us by now. No CB is going to be some magic wand to fix that and suddenly bring us back to the treble winning years.
Also, negotiating itself is pretty difficult, we're not going to be able to land every player we go for. If a guy like Rosell can't get a top CB for a decent price, don't think that you could either. It's a difficult and complicated process, one that is often put to shame by unappreciative fans. Their decisions itself were not smart at all, but that's not 100% of the job is it? Maybe you could make the better decision of going for the more useful player, I doubt you could understand the rest of what has to be done.
While the board has been pretty blind in some areas and made some huge errors, do not for one second make the mistake of thinking that their job is easy. Compared to other clubs, they haven't done the best job. But it's way too far to act as if what they do is so easy that anyone could take over and do a better job. I'm not pleased with any of the management or board currently, I'm sure you aren't either. A lot of fans aren't. But that shouldn't be an incentive to claim that their duties are so simple that choosing between players is all they had to do and they got it wrong.