Make her an Aleut paddle.
Seriously, heat seems like a good suggestion although heat transfers so quickly through aluminum, shrinking and swelling the tubes with hot and cold may be difficult. The oxidized aluminum may have created an interference fit that is very strong.
Since you plan to shorten the paddle anyway you might try grinding a flat on the outter tube until you split it enough to help with separating the two tubes.
Although, you may just have an opportunity to carve another paddle.
PFD's (Life Jackets)
Gedi Convertible Helmet
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