What is the perception of the body? I'm following the four foundations of mindfulness. This has given me round-the-clock body awareness. It can often be painful; but it can often be pleasant. The pleasantness is marred by its opposite.
I notice from time to time the density of my body disappears along with all ideas of pleasant and unpleasant and their respective sensations.
Questions
Is the body a perception like all other perceptions?
How does the subtle perceptions described in MN 121 translate into the body? - I'm asking this as there appears to be greater mental/emotional turmoil like an archaic or ancient frustration/restlessness.
The body is obviously a perception (sense object) but it also exists independent of perception. For example, the automatic functions of the body (such as breathing, heart function, liver function, digestion function, growth, cell division, etc) exist and operate independent of perception.
Therefore, scriptures, such as MN 43, say the living body continues to exist when (living) perception ceases.
The question about MN 121 appears irrelevant. MN 121 ultimately says in the supreme realization/experience of Emptiness, the body continues to exist. It says:

‘This mode of perception is empty of the effluent of sensuality, empty of the effluent of [ego] becoming, empty of the effluent of ignorance. And there is just this non-emptiness: that
connected with the six sensory spheres, dependent on this very body
with life as its condition.’

In your attempt to practise Satipatthana, there is greater mental/emotional turmoil like an archaic or ancient frustration/restlessness because of your past kamma (actions). Restlessness is one of the Five Hindrances to meditation and the scriptures (AN 10.61) say the food (nutriment) of the Five Hindrances is the Three Unskillful Actions.
Instead of focusing on higher teachings, such as perception &amp; Emptiness, you should focus on teachings for rank beginners, such as the Five and Eight Precepts, which include not watching pornography, not engaging in casual sex and, on a higher level, not masturbating.
Satipatthana is the 7th factor of the Noble Eightfold Path, which includes abstinence from all sexual activity in its 5th Factor. If a person actually is practising Satipatthana &quot;around the clock&quot; (as you claim) then they will be free from all sexual activity because Satipatthana means giving up all lust and distress towards the world.
Your experience of restlessness is like a detoxification. When a person enters into detox via abstinence, they will experience withdrawal symptoms for a time, until they fully detox. A person cannot detoxify if they keep taking toxins.