Which would be correct? "He is here with us today to show us the way." or "He is here with us, today, to show us the way." Why, exactly?
I would say the first sentence is fine, though I would lose the second "us". If you say this aloud, I don't hear the pauses where you have placed the commas in the second sentence.
Both are correct; however the importance of the word "today" is diminished in the second sentence. When you put a set of commas around a word or phrase, it becomes a parenthetical element, something that is unnecessary to understand the meaning of the sentence and can be dropped. When you are writing, consider whether certain information is important or additive. If the information is important to the understanding of a sentence, then try not to wrap it around a set of commas. In your examples, if you think it is important for the reader to be aware of the specific time, use the first sentence. If you think it is not important for the reader to know the specific time, use the second sentence or drop "today" completely.
i don't see any need for a comma before today, though the one after it is necessary... and either the first or second 'us' is unnecessary and annoying... choose one and dump the other... or dump both, since neither one is really vital to the sense of the sentence... 'here' suffices to let us know where 'he' is...
because what comes after it is a modifying prepositional phrase that needs to be set off by a comma... it tells why 'he' is 'with us'...
The first one to me makes more sense. It sounds as if you use an apositive in the second witch you do not.
This reads best to me as well. Take out "with us today" because it is implicit. If not it would be something like will be here or has been here. But "is here" denotes at this very moment.
Yes, the "today" is redundant and you are better off without it, unless: he was here yesterday or will be here tomorrow for some other reason; or this is dialog and you want to show that the speaker is long-winded. In neither case would I offset it with commas.