Andrew R Stewart 

I don't know why one might think an AL ebike frame might be "very nice". The vast majority of those I see are simple thick walled, built for a price point constructs made with various designs to deal with the non human power and weight that aren't needed for you or I. The others are either worse or way too expensive for the first owner to just let go without some thought.

As to the corrosion: who knows from a couple of photos. But since you have the frame anyway why not go ahead with your plans and report back on your results. At worst the frame will crack/break, at best you'll get many miles of function (note I didn't say "fun"). You will want to remove the corrosion completely enough to reach clean and smooth metal, or else the cancer will return even sooner. Monitor the areas over time. I have less concerns about safety as the chain stays are still held in place by the unbroken other ends. Complete fractures in these locations generally result in some funny bike feelings and you are able to slow and stop with little drama. (Forks are a very different concern!). Andy