You wrote, “the Hebrew word for “blood” which is “Dam” but in this verse, the word is “damei” This would be plural form, or in English “bloods”.” I would like to know what your source is for this statement.
According to Strong’s, the word is “Dam”, #01818, and is found 361 times in the OT. In 342 cases it is translated as ‘blood’. Strong’s says the word is a masculine noun but does not state if it is plural or singular. But, I would think, blood is a word that is never used in a plural form. A numeral is never placed in front of that word. You can’t have 1 blood or 2 bloods.
I think you are trying to get more out of the scripture than is really there. When you speak about potential offspring of Abel, you seem to imply God did not know ahead of time that Abel was going to be murdered. God knows all, He knew Able would be killed and would have no descendents. So, to answer your question, the Bible tells us about one murder that Cain committed.
I totally agree with your closing comment, “Life is precious, and eternal life in Heaven even more, are you sure that your eternal life will be in Heaven?”