The 1st sounds like "Mutant" (1983). The younger brother gets dragged under the bed.http://www.badmovies.org/movies/mutant/
For the second, I wonder if you could have seen "The Brain Eaters" (1958). The others that seem probable:
"The Crawling Eye"
"It Conquered the World"
"Zontar: The Thing from Venus"
I don't remember the threat of slit wrists in any of them, but the general plot fits.