Umm no cause the tailed beasts are not obitos power that's why. That's why they are not here. Not obitos power simple as that. If we put in the beasts then it wouldn't be fair, cause obito can control them, Madara can control them and Nagato can control them, and who would summon the gedou? All 3 of them can so it would be stupid.
No it said Rinnegan and susanoo combined brought them down, and the only thing that makes sense is bansho tenin. So it was not susanoo alone it was both susanoo and Rinnegan. And no you can't even give a reason why Obito without the beasts can beat Nagato. Nagato>Obito
Yes Obito fought all of them but guess what he couldn't harm them either! Notice Obito has to be tangible to attack and when he turns tangible he is vulnerable to attack. If you reread the fight, kakashi, guy and BM naruto were just fine, it was more of a stalemate until kakashi figured out how to hit him.