Every time something different :-> . There is simply no (agreed) definition of intelligence.
Not really. The relatively biggest brain (that also needs the largest percentage of oxygen) of all animals can be found in an African freshwater fish ("elephant-nose fish") that has so far not shown any particular intelligent behaviour.
Strictly speaking, yes, the brain makes up 10 % of the mouse's weight, but 0.1 % of that of the elephant. But this ratio is itself dependent on body mass, and if you take this into consideration, both mice and elephants are very precisely on the regression line ( ~ average) for mammals (some mouse species are higher, though, more like humans and dolphins).
I don't think so
intensively discussed in the already mentioned thread in May
Definitely. We can't possibly test that, but nerve signals have the same speed in all living vertebrates, so there's no reason extinct ones should have been different.