5. “two posts side by side on one page“. – This would mean 2 posts side by side on one POST ( as a page is technically a post in publii). So, even in WordPress, this is not possible as far as I know. But if you talk about two Blocks side by side, it is possible ( example: https://oxlin.io/contact-us.html)
7. Mega Menu – No. I asked Bob while I was evaluating Publii, he confirmed it. I think they want to keep the app as simple as possible.