First, how much organic molecules in the clouds?
From
The Physics of the Interstellar Medium by J.E. Dyson & D.A. Williams (Institute of Physics, 1997), p. 31, the number density of carbon monoxide (CO) is about 10 millionths of the number density of molecular hydrogen. This implies
N(CO) ~ 10,000 to ten million molecules/m³ in dense clouds. For a dense cloud with a mass of 500 solar masses, this implies a CO mass of about 2500 Earth masses (8 Jupiter masses, or 200,000 lunar masses).
G.A. Blake
et al. (1987),
Astrophysical Journal, 315, 621-645 -
http://adsabs.harvard.edu/full/1987ApJ...315..621B7 - find a column density for CH3OH in the Orion molecular cloud (about 500 solar masses) about 1/1000 of the column density for CO, and column densities for C3H2 and CH3CN of about 1 to 10 millionths of the CO column density. This would yield roughly 2.5 Earth masses of CH3OH, and 1/5 to 2 lunar masses of C3H2 and CH3CN.
Second, BEFORE the molecules are adsorbed and protected, what is the chance for an organic molecule to survive?
Dyson and Williams (p.36) say, 'Ultraviolet photodissociation occurs for most molecules unshielded by grains in a few hundred years.' Obviously interstellar organic compounds survive long enough after their formation to reach large enough abundances to be detected by radio telescopes.
There may be plants that already exist before the planet was destroyed by the star.
Where do you think that interstellar clouds come from? They are formed by compression of the diffuse interstellar medium; giant molecular clouds have masses of thousands to millions of solar masses, far larger than any planetary system. As I have already explained, dense molecular clouds can contain a lunar mass or an Earth mass of such molecules as CH3CN and CH3OH, far more than the total mass of plants on any known planet.
To make the point simple: the organic material embedded in comets must have been a larger group of molecules, rather than smaller group of molecules before joined the comet. Gravitational attraction works very slowly. Most organic material should be destroyed in space very quickly.
No, I think that you are wrong. From
The Physics of the Interstellar Medium (pp. 39-42), it appears that H2 molecules are formed by catalysis on the surfaces of cold grains, and other molecules are synthesised by 'networks of ion-molecule reactions ... driven by the ionization produced by cosmic rays.' Once formed, the molecules can be adsorbed onto the surfaces of the cold interstellar grains.
Again, you must remember that interstellar clouds are opaque, so that molecules in their interiors are protected from the ultraviolet radiation that destroys molecules in their surface layers. In fact, the destruction of molecules near to the surface of the clouds protects the interiors of the clouds by absorbing the high-energy radiation.
I apologise for the long delay in answering your post. I hope that I have explained the physics of interstellar clouds clearly enough for you to understand at least partially how molecules are formed and how they survive in these clouds.