Concerning the Summoning, in my opinion, i haven't seen the correct use being metioned yet.
Ok, they reduced 3 CCP to D3 CCP per spell, but they buffed the rest.
Old book you used your Slann spells ( 3 ) for 9 CCP
New book it says - as one of you already pointed - EACH SLANN of ORACLE.
So if I have a Slann, Kroak and an oracle, the 3 of them can use this ability.
What I haven't seen yet being mentioned is the following:
RAW: ...before attempting to cast the first spell ... carry out cellestial conjuration ... if so, D3 CCP and reduce spells by 1
Which in my opinion means: As long as neither of your spellcasters has started to cast a spell they can keep on trading spells for CCP
so, slann can trade his 3 spells for 3D3 CCP, kroak can trade his spells for 4D3 CCP, etc.
So in a game with slann, kroak, astrolith and oracle i could end up with :
D3 ( general ) + D3 astrolith + 3D3 ( slann ) + 4D3 ( kroak ) + D3 ( oracle )
Which means at least 10 CCP and potentially 30 CCP ( if reaaaally lucky

)
30CCP aka first turn 50 skinks or an engine summoned
( which would also make the Dread Saurian cost more logical, why bother setting him to 50CCP if it was impossible to reach that in any given game )
Do correct me if i'm wrong