“God knows their hearts. We don’t.”
Sure we do. If they tell us that they are Muslims, we should believe them that they are Muslims. No Muslim has ever gone to heaven and ever will, as salvation is through Jesus Christ alone.
How does the second clause relate to the first?
If a Mohammedan has never been exposed to Jesus, either through evangelization or reading, yet lives a good life, and pursues truth, to the best of his ability, would a just God condemn him to hell for eternity?
Yes, anyone who is saved is saved through Christ's redemptive death on the Cross, but it doesn't logically follow that only those with an explicit faith in Christ will be saved.