Hassle free lifetime guarantee, they say

That's bullshit. They shouldn't advertise something they're so reluctant to provide.

For what it's worth, I recently broke the tip off a small Craftsman screwdriver that they no longer sell as a single. They gave me the whole set that it now comes in, no questions asked. I was pleasantly surprised.

That's exactly how it should be done. The pack they would have cut open would become worthless and not doing anything would piss off a customer. Granting the action the way they did secured a happy repeat customer.

I remember when I was a late teen, there was a news segment in Minneapolis that was covering customer service issues people were seeing.

Some guy went to 4 or 5 big name department stores in downtown Mpls. He would tell the sales person he was in town for a very important meeting and his wife packed the wrong shirt, that was the wrong size. Problem was the shirt wasn't from the places he went to return it. He said, if you'll take this one shirt back I'll buy 5 so I don't have to worry about this again. Every time he was denied, until the last time. The manager happened to hear him just a the sales person was saying no, the manager walks over and says he'll take the return. The "customer" puts his brand new, in the package shirt on the counter, buys 5 new shirts. When they are done, he asks what they'll do with the original since it's not something they sell. The manager picks it up and tosses it in the trash. His response was something to the effect of "It's trash but it helped me hopefully win a new customer". It's important to note, they gave him a fair value for the shirt.

Some managers aren't lazy and can actually do math. Some managers just like to exercise "power" over anyone they can and forget customers have the power. Same goes for clerks.

I think you handled your situation perfectly. It's not a fight with the clerk. They are "probably" doing what they were told, or at least trying to show some sort of company loyalty and not letting someone "scam" them. It's not their job to 'make decisions".