Not really simple answers to your questions - StorJ documentation says yes Public Access to uploads is possible, yet not exactly straightforward.
(I’m not an AWS user, so I could not compare the two).
Your second question I may not fully understand, “downloaded” without a key - technically we would only link to a file? but the link would be generated using a key (at least that’s my thought process on this), not sure you would be able to download.
StorJ public access to buckets
Even looking at those I’m not sure if they can be incorporated into discourse.
(while looking into this, I see that Storj is using discourse for their forum, maybe I should ask how they are handling uploads..)