Good for you, nice collection...
Elohim אלוהים is an obvious plural noun in Hebrew, so ancient Hebraics postulated that the plural declension was an indication of reverence.
In Arabic, Allahum اللّهم does not look plural, so Arabs postulated that the M (meem) at the end of the word Allah was a substitute for the lacking vocative article, which does make a bit of sense.
The other explanations are modern ones. I don't know if Elohim was truly just a way of revering El, which is very possible, or if it had another relation to the pagan past of Hebraics. However, if I were to assume something, it would be that the word simply relates to the fact that ancient Semites were polytheists. This makes much more sense to me than the funny explanation you cited.
Actually what you said about the history of Hebrew and Arabic is true, but it is not why Hebrew is simpler than Arabic.
Formal Arabic is much older chronologically than Hebrew. Although its origin goes back to thousands of years ago, Hebrew today amazingly resembles the modern informal Arabic dialects, especially the nomadic dialects. As it is natural, those dialects have lost most of the complexity of ancient Arabic grammar. So modern Hebrew is less complicated because it is much more evolved than formal Arabic.
Formal Arabic has not changed a bit for 1500 years; this has been actively maintained for religious then political reasons.