I'd say 7 - 10 aren't totally outside the realm of possibility.
#7, Space Marines do suffer cognitive dissonance when the right situation arises. Being ordered to do horrible things in the name of the Imperium, being ordered to act in a manner counter to Chapter doctrine (either by the Chapter or other forces). And, of course, the corrupting power of Chaos. Sometimes just remaining true to your understanding of principle can turn one into a heretic, if it disagrees with what the larger Imperial authority wishes or believes. Some common themes are: victory at any cost as an act of heresy, refusal to carry out an unconscionable order as an act of heresy and desertion as an act of heresy.
#8, any number of situations could make this possible. The easiest of which is just your squad assuming you died in some catastrophic incident like an orbital strike, cut-off from the squad in battle, a drop pod hit during descent or that malfunctions and goes off course, lost in the Warp, ect...
#9, actually pretty likely. Rogue Traders are, among all Imperial citizens, the most likely to embrace things "outside of Imperial law." I'd think a former Space Marine's motivation for playing body guard to a rogue trader is more important than whether any rogue trader would conceivably retain them.
#10, There's a gut instinct, I believe, to view the Imperium as All-Seeing and All-Knowing. But the fluff goes on, and on, and on about how there's millions of worlds, billions upon billions of people, long-range communication is slow and at times unreliable, and travel is abysmally unpredictable. Now, the Space Marines are separate from the Imperial organs of governance and have a smaller clique to keep track of. But if you take into account the possible situations above...really the only way a Space Marine's former chapter would know they'd gone rogue (instead of dying) is: meeting them, hearing about them or having a Psyker do some sort of reading.
That said, in all the 40k reading I've ever done, post-heresy, there's never been an instance of a traitor actually making into the ranks of the Space Marines as an initiate. The screening is simply process simply goes on too long, and is incredibly invasive.