Brilliant, I was also thinking about a wheel and teeth set up like the second file. I hadn't gotten around to thinking about how to release the shaft yet, I like the idea of raising the cog. I like your design much better than the AcuSpear because it would be the perfect way to incorporate rubber shooting bands like most spearguns, spear poles and hawaiian slings use. Band material is cheap, durable and they would be much easier to rig up. The red lines/string(?) in your second file would be how I'd probably try to rig up the bands.
As far as the spear tip I don't think a spring loaded tip will be necessary, there are double flopper tips like you've drawn but many, including myself, prefer single floppers for spearfishing a single flopper is very reliable. Either styles would work. The double floppers have a retaining ring that keeps them closed until it contacts the fish and a compressed piece of rubber helps spread them.
Double flopper: http://a248.e.akamai.net/origin-cdn.volusion.com/psrbn.xmcnq/v/vspf...
Single flopper: http://flfreedivers.com/media/catalog/product/cache/1/image/9ade485...
It would be easier for the fish to slide down the shaft with the single flopper that is riveted to the shaft like the picture so that would also be a plus.
As far as dealing with live fish on the shaft, I'm not really sure what would be best. Personally since they are a devastating/invasive species I might even consider forgoing the flopper so that the fish are not retained. But I don't really care for wasting perfectly good meat so... maybe not.
I think I'll grab my speargun and pole spear and ponder this with a beer tonight.