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[anagogy]

Do our bodies have sentience, a subjective experience of their own?

They have a subjective experience but it is merged with your subjective experience. Each had a subjective experience prior to joining, and after joining, there is a merged one. This is why bodily impulses feel like "your" impulses. At death, they separate again.

Does the body suffer? Does it only conduct and transmit our pain, or writhe in agony as much as its user?

Both experience it as long as both are together, or focused in/as the body. If both leave the body, then the material of the body will degrade back into its 1st density elemental constituents. In other words, the body dies, and the various aggregates of consciousness go their separate merry ways. The native dwelling place of 2nd density consciousness is the lower astral plane (the thought-form level), and the native dwelling place of 3rd density consciousness is the middle astral plane. 1st density consciousness doesn't have much of a separation between time/space and space/time. In that way, it is similar to 7th density.

Does the chemical vehicle resent it's wearer for neglecting or abusing it?

No. It's not capable of judgment in that way. It mostly just instinctually reacts to things in the direction of safety and comfort.

Is an overweight man, or a hard drug junkie, or a self-harming woman hated by their own flesh?

Ditto last answer.

What if the musculature of a bodybuilder want to stop being strained well beyond its capacity just to satisfy the vanity of its user?

The body's consciousness, of itself, is not ego driven, only the 3rd density mind is. But as I said before, they are one for the duration of the incarnation.

Do the sexual organs of a religious monk resent him for being denied relief?

Ditto.

When someone hits himself out of anger, does his body likewise become angry towards him?

Ditto.

No doubt our bodies are alive, but to what extent? Is the body capable of feeling under-appreciated?

It doesn't feel underappreciated unless you personally feel under appreciated.

2nd density bodies can exist without 3rd density souls, just look at most of the animal kingdom. Even human bodies could live without 3rd density consciousness, but they would be animalistic and completely driven by the senses.