The pineal gland and every other living cell in your body needs oxygen without it those cells cannot function. therefore no the pineal is not seperate from the rest of the body and is certainly not alien its quite human take out some of its DNA compare it to the rest of your body and its human.

although serotonin is very involved in perception its not the only or the major neurotransmitter. things like acetylcholine, endegenous cannabinoids, dopamine, gaba, endorphins etc etc are all playing an important role in allowing our mind to communicate with the rest of our body and interpret the signals coming from the outside world. we are tripping on them though if you look at it like that sure

its also important to note that things like LSD can only be detected in the body for an hour or so after that its gone its already done its major effect. the rest of the trip actually seems to be a result of the downstream effects of the agonism of LSD at 5-HT2 receptors. therefore they are not by any means replacing the neurotransmitters in your body (unlike things like opiates) they are just changing what you already have a brief tweak in the system causes the wonderful beautiful mess we call tripping.