How do you know what is widely supported by the thread? Do you just have to manually slog through everyone's posts to find commonalities?
How do you get people to engage in and continue conversations about persuading them about things?
The most effective way of understanding what the thread supports is to follow discussion in the thread in real time as well as just reading updates. So, yes, manually slogging through everyone's posts - but doing it in real time instead of in huge blocks, until over time you build up an impression of what people's attitudes are. Honestly, I found doing that really interesting even when I was a reader - this community generally produces discussion of comparable interest to the updates themselves, at least if you're invested in the topic, and if you're not invested in the topic you're probably not coming up with ideas about it so you can just skim over posts in that area.
As for getting people to engage in and continue conversations, IMO probably the most important thing is to use the reply function. For example, in the discussion above, you're replying to
@T_of_A and
@Sir Stompy, but you haven't actually quoted their posts, so they won't get pinged. In some cases, your interlocutor might see what you said anyway, but often - especially if the thread turns over to a new page - they just won't notice that you said anything. Quoting their post pings them (meaning they'll get an alert that you replied to them and are much less likely to miss it), and also makes it much easier for
other people who might want to jump in to find out what the conversation is about, since they can look at the post you're replying to at the same time as your reply, and if they want they can easily follow the chain of replies back to catch up on the whole conversation.