For the second, you cant follow accounts on lemmy but you can follow lemmy accounts from mastodon. (it will show both posts and comments the person makes as boosts)
Same for posting, you cant post so it shows up on mastodon since lemmy doesnt have hashtags but you can post and comment from mastodon (post by mentioning the community in the post and comment by replying to a post made on lemmy). Note on posting from lemmy to mastodon though is if you have a mastodon account that follows the lemmy account and comments on posts with hashtags that comment and the parent post will appear in those hashtag feeds, just looks spammy