Jeremy you have posted 3 times and none of the posts say anything about a limit to extending 30 day exemption by land stamps. Steve is quite right and you have misunderstood all of the posts. They say only 1 X 30 day extension is permitted for each exemption either by land or air....max of 2 entries by land. If you insist that you are correct and can find some solid evidence that states that the 2nd exemption stamp cannot be extended then all good. But you can't because there is none because the rule does not exist. You can extend 2 times....end of comments. Unfollowing this post as you don't seem to get it.
The proof that it is possible is discussed in the article you posted. Because there is no Statement whatsoever that states otherwise. Your own information states that 2 x land entries are permittable and all 30 day exemptions can be extended by 30 days. There is no information anywhere that supports your claim because it is incorrect. BUT repeatedly crossing borders is never a good idea and you will eventually get pulled up by the imm police. Visa is best option for long stays...and rightly so if you are no longer a tourist.
There is no rules that I am aware of, or have found, that say you cannot extend a 2nd 30 day exempt by land. There are no sources. That is my point. I am not sure on anything unless I see a source that proves you CAN'T. Which you did not provide and I can't provide either.