On the first point, propagation happens passively too. For example, if someone follows me on Mastodon, this reply will be pulled to his/her feed as a microblogging post, and will be discoverable on any feeds my account later appears. I remember also testing around between Lemmy and Mbin how liking/upvoting works, and doing that on Lemmy while the Mbin account followed it also showed posts previously not on my Mbin instance.

Similarly, Mbin (and dunno about Lemmy but I'd imagine it's the same) pulls Peertube channels as magazines/communities, and iirc Peertube channels can also be followed as users on microblogging platforms, and in both cases their video uploads automatically appear on the respective text feeds.

And about the second point, I agree, but that's also why I suggest recommending major platforms. For example, Lemmy.World and Mastodon.Social are likely to have far more publications being posted on or propagated to than Mbin or PieFed instances, since the former two are around for much longer, and not actively trying to be toxic (at least from what I can observe).