(a) No. Electrical forces depend upon the nature of the intervening medium while the gravitational forces don't depend upon the nature of the intervening medium. So, such shielding acts are not possible in case of gravitation i.e., gravity screens are not possible.
(b) Yes, Astronaut can hope to detect gravity if the size of th e spaceship is extremely large, then the magnitude of the gravity will become appreciable and hence the gravitational effect of the spaceship may been me measurable.
(c) Earth moon distance is very small as compared to earth-sun distance. Tidal effect is inversely proportional to the cube of the distance it means it is not governed by inverse square law like the gravitational force (which obeys inverse square law). Hence, tidal effect of moon is larger than that due to the sun.