These days, I'm much more inclined to say "What if..." or "We should also consider..." to preface an opinion, such that consideration is required before dismissing what I've said.
I try to make myself do this even when I'm not arguing with someone. If a task comes to me from my boss, I will do my best to rephrase it in my own words (not verbatim, unless it's something really simple) to make sure we're on the same page.
Don't worry, even if you don't say "yeah, but", I still think that.
I've experimented with "What if" too. I imagine it works for others, but for me, and perhaps something about my tone of voice that I'm not realizing, it sounds patronizing, so it hasn't worked as well.
In that scenario "I think" is not to do with opinion but uncertainty (it can be used for both).
The definition of Linux is an established fact so if you know what it is there is no need for "I think".
So, you agree Linux is just a kernel and the whole operating system should be called GNU/Linux, right?
For the third scenario, "it appears" can be useful, or if more justification is required, "it appears from the (server's behavior|available data|documentation I can find)." Then when you have to correct yourself later, it wasn't you who was misleading, but the situation. I find that this can be used in a completely honest way and often gives non-techy people a more accurate impression of the situation. It helps them understand both the tentative nature of your diagnosis and the obstacles you're struggling with. Win-win.