Overestimating Alexander's ability at this point. Prussia isn't going to tolerate Napoleon's son on the throne right next door nor was anyone all that enthusiastic over Bernadotte. Ultimately the Bourbons were the best choice out of many bad choices.
Alexander's, yes (you need a much less Anglophilic Alexander to push the schema through). But you are seemingly overestimating the British abilities (no army to support their demands if push came to shove) and underestimating Prussian dependence upon Russian support of _their_ demands. With Austria on board Russia could guarantee Prussia's demands regarding Saxony in exchange for supporting Napoleon II.
With Bernadotte the issue of support was more complicated because Austrians considered him at least somewhat "revolutionary" and because, after figuring out that his claim to the throne of France dies not have too much of support among the Allies he concentrated on minimizing Swedish losses (sometimes up to a degree which frustrated the Swedish military looking for the glory), possibility to get Norway as a compensation for Finland and later for getting money for Sweden and its royal family (money always were his big concern). However, if he got a firm support of his plans he could be much more active in 1813 - 14 gaining more points with the Allies (even in OTL he won battles of
Großbeeren and
Dennewitz and went his own way only after Leipzig). Of course, this would not endear him to the French, at least initially but Louis XVIII and his family were not exactly everybody's darlings either as was demonstrated during the 100 Days.
There was also Eugene who would have some benefits of Napoleon's clout while not being blood relative (if this was such a taboo). He was a capable person, generally popular with everybody and marriage to Wittelsbach provided him with the impeccable "royal credentials".