Nirvana is cessation. Para means "beyond," so Paranirvana is beyond Nirvana, and Maha means "great," signifying "the great beyond Nirvana."
What matters is the consciousness, not so much you bodily position. However, sleeping on one's stomach is not so great for the spine, since the pressure on the cervical vertebrae can produce back pain, neck problems, and even headaches. It is also better for your breathing to sleep either on your left side (as taught in Ayurvedic traditions) or back. Samael Aun Weor explained that the dorsal decubitus position, such as with the statue of Chac Mool, is conducive towards astral projection when in full concentration and relaxation.
When you think of astral projecting, don't let any other daydreams, preoccupations, worries or ideas enter your mind. Just focus on falling asleep and remembering your Being; don't think of other things. Concentrate on being aware!
Single-pointed concentration is the ability to focus on the object of meditation without getting lost in thoughts and contrary ideas. It means to simply look at, with your spiritual sight, on a given object, whether it is through retrospection meditation on the day, focusing on a given event and the egos that manifested, or a holy and religious figure. You just focus on that object and do not let your mind take you off to other things. This is the challenge!
Regarding your question of mantras, yes, that is what it means to concentrate on the mantra. Pronounce it, feel its energy and vibration, and don't think of anything else than what you're doing!
Single-pointed concentration is developed the more we meditate and work in self-observation, remembering our Being and never falling into unconsciousness of the psyche in our daily life.
Joyful in hope, suffering in tribulation, be thou constant in thy prayer.
Benedictis, qui venit in nomine Domini. Osanna in excelsis.
"Blessed is he who comes in the name of the Lord. Hosanna in the highest!"