How to protect people walking around the deck from banging their head on this overhanging platform

You didn't say where you are located, but i figured in a cold area like me in Maine. First of all, 12 inch joist spacing a a waste of time and money. 16 inches will be fine especially if you plan to use 5/4 decking of any type. If you are in a non ground freeze area, just use simple blocking to support the additional 4 foot off the concrete. Assuming you are in the frozen north, consider using 3 foot porch posts. they are pre-poured concrete pyramids with a threaded insert dead center on the top. Often easier that sonotubes and mixing concrete. If your existing slab is not underfooted with a frost wall, and has not heaved or cracked over the years, then most likely a simple 8 inch concrete block in the ground the same depth or slightly deeper than the slab will probably do just fine. Remember you are only supporting 2 foot of deck (2 ft supported by slab and 2 ft supported my new foundation) and the decking itself will be spanning 8 to 12 feet onto the existing slab. Over kill is fine, always safe, but not always a good use of time and money. Also if you new deck is 12 inches off the ground, please consider some safe egress steps and side barriers. The rise of the step should not exceed 8 1/2 inches. A 12 inch step down can be a hazard, especially at night and for younger and older folks. Check your local code or the new IRC guide lines.

If I understand your description correctly, you have a maximal load area of ~38 ft². If so, 4x4s are more than sufficient for the posts. (Check your trusted load tables; don't believe some random guy on the Internet.) If you replace some posts there is no harm in upgrading them to 6x6s, but I don't see a reason to sister onto the existing 4x4s.

However, it sounds like there is so much rot that it will be easier to tear down the entire deck and build a new one. If you do that, the new deck will of course need to meet current code requirements.