1. Ishbi-benob, killed by Abishai
2. Saph, who fell to Sibbecai
3. Elananan’s victim.
4. The 24 digited giant slain by Jonathan

If you look at Chronicles, Ishbi-benob is not mentioned. and Saph is called Sippai. So in this passage, the Chronicler mentions three giants where Samuel mentions 4. I don’t see how the Chronicler is adding to Samuel’s account. Instead, he seems to be leaving one out.

Or is it you contention that he left Ishbi-benob out because he added Lahmi. If so, how is his omission of the count relevant?

Now, on the “brother of” issue, you seem to be arguing thus:

1. In Samuel, four giants are mentioned who are “born to the giant in Gath”, i.e. they have a single common parent.
2. Since the giants have a common parent, they must be brothers.
3. If one of these giants is Lahmi, the brother of Goliath, then the other giants must also be brothers of Goliath.
4. It sounds odd for only Lahmi to be called out as a brother of Goliath and not the others.
5. So the chronicler must have added the “brother of” bit to ‘fix’ the account (rather than “Lahmi, brother of” being corrupted to “Bethlehemite” by some combination of manuscript damage and copyist errors along the way).

There are two problems with this argument.

First, even if the text forces us to accept that the giants had a single common parent, Lahmi might be the only full legitimate brother of Goliath killed by David and his men. The others might be half-brothers and even bastards.

Second, the phrase “born to the giant” could have been translated “descended from giants”. There is no definite article in the original. The word Rapha (giant) appears in exactly the same form in both passages. Various translators translate the two passages variously and inconsistently as singular and as plural. This undercuts the idea that the text requires that the giants have a common parent.

Note that both accounts say all the listed warriors were born to the giant in Gath (2 Sam. 21:22, 1 Chr. 20:8). If “Lahmi” was Goliath’s brother and they were all born to the giant, that means they were all the brothers of Goliath. To write about four brothers of Goliath but refer to only one of them as such would make no sense. Thus, 2 Sam. 21:19 makes sense only if it refers not to a brother of Goliath but rather to Goliath himself, which means it is saying that Elhanan is the one who slew him.

This would also explain why the Chronicler omits the specific number of those born to the giant in verse 8. He was adding to that number with his invention of “Lahmi”.

Again there was war with the Philistines; and Elhanan son of Jair killed Lahmi the brother of Goliath the Gittite.

I think it’s not unlikely that the verse in 2 Samuel has just been corrupted over time.