Is it possible to extract individual timestamps of the transactions' execution time in a given block? The Ether-scan API gives only the timestamps of the block collation, doesn't mention anything about individual timestamps.
No, because such a thing does not exist.
It's not that the data isn't there. There is nothing to record. It may help to think of the transactions as well-ordered (this came before that) but instantaneous.
A miner solves a block at a certain time and nodes learn about it later. Yes, it takes a certain amount of time for a node to work out the effects of a transaction, but that work is undertaken after the fact and non-simultaneously by each full node. The only reference to temporal time we have is the block timeStamp which is somewhat subjective (caution) since it is placed there by the miner.
Hope it helps.