For a Core i7 under great load, late 70s is fine. 80s is okay but not recommended at all. But unless you play several hours a day and intend to use that CPU daily for 2-3+ years, there isn't much of a risk.
Ideal is as low as possible. Preferably 60s or so under full load or lower, however, that isn't absolutely essential.
Many people with high overclocks get in 70s, and frankly if it is only occasional then even 80s would be fine.
But don't touch 90, better to get better cooling before that.
But 75 is fine, although a good cooler will bring it to about 50C at full load at stock speeds and sub 75C under overclocked settings
Yes, but with any aftermarket cooler that too at stock, 50s is the max I would say you would get. or at most lower 60s in a hot country. now that isn't essential, but any good cooler would ensure that which would mean more margin for overclocking