Ok, not using featured image but rather inserting image into posts, solves the double image issue. Solution is very similar like when using featured image on Lite version, image shows up in full view and category/post summary page.
Aside format post doesn’t show post title in category/post summary page and full view by intent, right? No “edit” button either?
Regarding the error, am getting to aside format post in 2 ways: via main page link to post created by “Posts for Page Plugin”. Both give the same error but also opening the link directly. Both give the same error. Additionally, using breadcrumbs links.
I did what you proposed and edited content-aside.php and it worked. No more error.
Also, the quick tutorial did the job. But, how can I get the grey rectangle background for the featured image that stretched over the width of page? Like it is in the Lite version. I kind of liked how this background emphasized the featured image.