With regards to needing a bill of sale, they did not require it of my Squirt Es4, when I sent it in. (scissor spring kept snapping off to the side)
The thing is, I don't think they have any models yet that have been made for 25 years, so they haven't had to ask for a bill of sale. Also, someone above said they are date stamped. In the absence of a receipt, that would be used to determine the tool's age. (a tool could sit on the shelf of a retailer for a couple of years before being sold, so the receipt is best if you want to hedge your bets)
When they get to the point that they have a model that has been in production for 25+ years, we will see what happens.
My PST is probably 20 years old now, but Leatherman knows that, as it is discontinued. If I broke it, they would likely replace it with whatever their basic, non-one-handed blade model is these days. They have a clause in their warranty that states that too. They leave it up to their option, but I bet they don't do much repair of MTs. It doesn't seem like it'd be cost effective for them. They probably just take the old broken ones and throw them in a pile to be recycled.