I'm not sure you should bother trying to free the joint and just treat it as a complete entity. Metals respond to force in rather predictable ways and it seems quite likely that the existing weld is probably stronger than the shaft. I haven't worked all that much with aluminum, but know that it is reasonably easy to deform it (which may be part of why it isn't coming apart).
If making the paddle shorter is the goal, it'd be easier to cut off the desired amount from each end and then reattach the blades.
Or better, if money isn't any issue, replace the paddle with something of higher quality since it is a huge part of the paddling experience.
Kayak Motor Kit
Classic Freestanding Rack
Kayak & Canoe Covers
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