pack a hefty punch in terms of cerebral cool.
There are three ideas here:
1) to "pack a punch" means to have power, often hidden power. "Packing" means to store or carry something, so someone who packs a punch is prepared to deliver that punch. In this sense, the punch isn't necessarily violence, but something that is powerful, surprising, and explosive. For instance, I might say "That hot sauce really packed a punch!"
2) Cerebral means having to do with intelligence or the mind. In the context of films, it means that the film requires and/or deserves thought. The author contrasts films that are expensive spectacles with less-expensive films and points out that the cheaper films are still cerebral.
3) Cerebral cool is an unusual phrase. Cool means something that is interesting, appealing, and mysterious. To judge something in terms of "Cerebral cool" means it must be appealing to those who want to really think deeply about it.
Putting all three ideas together means a film which brings an unexpected, powerful thrill for someone interested in a smart film.
As Emmett and Michelle ... adapt to their ... life without getting (homesick OR the blues)
Here we are talking about two people trying to adapt, without getting either of two unfortunate conditions. Homesickness is a longing to return to a more familiar place. "The blues" in this context means depression.
Is Howard really a psychotic, deranged bad guy or is he just a lonely old fellow in desperate need of some company, while god only knows what has blighted Earth.
This is pretty confusing. It makes a little more sense if we break it up:
Howard believes the Earth has been blighted by "God-only-knows-what".
Given that Howard believes that, but it might not be true, there are two possibilities:
It's possible Howard is a psychotic, deranged "bad guy".
It's also possible that Howard is innocent and right.
You had asked about the meaning of "God-only-knows-what". Much like "You-know-who" from the Harry Potter books, this is a phrase that stands in for a proper name. In context, it means it is unknown to all but God.