No, saying “these ones” or “those ones” is NOT grammatically incorrect, though many people believe the opposite to be true. Those particular phrases are often frowned upon (more so in US English than British English) but have been around for centuries in both formal and informal writing (government, linguistic, theatrical, etc. documents/ plays/ books). Though many might proclaim them to be incorrect, many linguists do not agree. The idea of those phrases being incorrect is actually relatively new from what I understand, only emerging within the last century or less.
In The Oxford History of Classical Reception in English Literature, for example, one chapter's author uses the phrase “these ones.”
To conclude, phrases such as “these ones” are NOT incorrect, but are largely stigmatized and disliked (which also does not make them incorrect, however much those people might wish that they were). They might be a bit redundant, but redundancy also does not make them incorrect.
This article from Grammarphobia covers this question with a vast assortment of historical and modern examples by scholars and other authors.
The OED does provide a separate definition for I guess; It is definition 6:
I guess: sometimes used, with playful moderation of statement, in reference to what the speaker regards as a fact or a secure inference. Hence colloq., orig. in the northern U.S. (sometimes with omission of the pronoun) = ‘I am pretty sure’.
The three earliest attested uses are from texts written by John Locke, suggesting that he might have popularized the sense with this kind of meaning. The earliest citation is from Some Thoughts Concerning Education, dated 1692, so it's not an incredibly recent idiomatic turn.
Hence, your intuition was correct when you wrote:
The impression I've gotten as a native speaker is that "I guess" doesn't literally mean "I guess"; it means something more like "I conclude".
As a final note, I guess isn't limited to this sense of the word implying something close to certainty. It could still be used in a phrase that meant an actual guess, but such a use would be distinguishable through context.
Jim: "I guess there are 150 jellybeans in the jar." (sense 1: a real guess)
Sally: "I counted, and there are 287 jellybeans."
Jim: "Oh. I guess I was wrong." (sense 6)
As you can probably imagine, had Jim replied with the present continuous tense of "guess," it would have seemed strange.
Jim: "Oh. I'm guessing I was wrong."
Sally (confused): "Yeah, you were definitely wrong."
This seems to be exactly the distinction you described in the question, so the answer is yes, you're on the mark.
Best Answer
I think the first one is correct. The second one could potentially mean you had only one weekly meeting.
The following sentence is equivalent to the first one.