1. Before Cain murdered his brother Abel, one can suppose that death, as we know it today, didnt cognitively exist. How, then, did cain know that upon inflicting pain on abel, the latter would die?
He had knowledge of death.It would be odd for Adam his father not to have mentioned any of the consequences of leaving Eden.
Gen3:19 In the sweat of thy face shalt thou eat bread, till thou return unto the ground; for out of it wast thou taken: for dust thou art, and unto dust shalt thou return.
2.It is recorded in the bible that heaven belongs to those whose names were written in the 'book of life' long before the world was created. Does this mean that, excluding free will, every soul's destiny is predestined
God's knowledge of future events does not negate free-will and hence there is no pre-destination.
read more here on free-will and God
3. Is heaven a physical place which can be subjected and analysed within the scope of science.
If physical you mean,material with a geographical position in the universe,most likely no.
can then the concept of time exist in a context devoid of matter(i.e time can only exist in presence of matter)
there is nothing like an entity called "time",which exists.Einstein showed through the theory of special relativity that Space and time are one thing called space-time.
If you bend space you bend time.None can exist without the other.So no space/matter, no time.
5. If God is all knowing, was it in order for Him to 'regret' having created people yet he foreknew they would rebel.
I don't see why God should not express emotion even if he knew.Eg if am told that my daughter will die in 3 weeks from cancer,does that mean on the day she dies i show no emotions because i knew she would die?
6.Bibilically, the world is thought to be about 6000 yrs old(?). However, God, as we think, has lived for trillions and trillions of years. Was it only 'recently' that he thought of creating man?
which part of the Bible explicitly says the earth is 6000 years?this is a view held by creationists who take the genesis account numbering of 6 days of creation and piece together another verse in the bible which says that "a day may be a 1000 days to God."
And God has not lived for trillions of years,he's eternal.timeless,always existed.So your question does not apply because asking when God decided to create is like asking what time was it before time?