In the first case, I guess it probably depends on your kind of forum? Personally I wouldn’t want to re-write an editorial title, as I think it’s too intrusive on the users content just for SEO, particularly when you could just change a title tag. There may also be on-site CTR benefits to having both (i.e. keeping users around). I know RedBull use editorial titles and SEO titles for that.
In the second one, I think that would definitely work for many of the cases. I do think you might want to speak differently to a community than to visitors coming from Google. For example suppose I had a thread:
“Let’s discuss the positives and negatives of migrating from Drupal to Wordpress”
From the forum point of view, I like the attitude and call to engage that comes from the question. If it turns into an amazing thread though for searches I might want to alter the SEO title to:
“What to be aware of migrating from Drupal to Wordpress”
It’s a more informational, searchable title, but I think less inviting from a community point of view. Having said that, I think your suggestion would also work for quite a few of those scenarios.
A third possible case that’s just occurred to me: if a forum puts qualifiers at the beginning of types of posts, perhaps to make things more searchable internally or immediately flag things for users:
[Screenshots Included] - 8 steps to retune a Gibson Guitar
[Stickied Post] - 8 steps to retune a Gibson Guitar