1-
Theoretically that type of load spread across 16 different VRAM chips should lead to fairly cool operation, so you shouldn't NEED those 4 heatsinks. BUT, I personally plan on using some extra heatsinks I have sitting around in the garage for my R9 290 (assuming they fit).
That said, if you opt to only use 12 heatsinks, be sure to put heatsinks on the 3-4 VRAM chips in the top left corner near the voltage regulator up there. Voltage regulators can get toasty, and those are close enough that the heat can easily spread to the VRAM chips nearby.
Another option I've seen people do is take two of the longer-thinner heatsinks, place them side by side, and then spread them across two VRAM chips. So two sets of two VRAM packages share two heatsinks a piece (hope that makes sense), and the remaining 12 use the 12 heatsinks included.
2-
Yes, but you'll need to reapply the thermal paste on the GPU.
Anyway, I'm currently waiting for the Accelero Xtreme III to come back in stock. Somehow all the retailers sold out during black Friday. Not sure what happened--perhaps a black Friday sale I missed? Hopefully the heatsinks I have will fit, but if not I'll just buy a pack.
Which reminds me, if it isn't too much trouble, can you measure the height - width - length of the VRAM heatsinks included in the Accelero kit? It would be very helpful, should I need to buy extra heatsinks. If you are interested,
FrozenCPU.com has a nice selection.