Thank you for playing all the way through, and for writing all this up. This is exactly the kind of feedback that helps.
You're right about the vibrating table. Once screws, nuts, bolts and nails each had their own lane, the only thing left for the last lane was oddballs. So the fifth upgrade changed nothing and just cost money. The table now has 4 levels. If you already bought the fifth, you'll get those coins back when your save loads.
On the keepsakes: they aren't actually random. Each one is tied to a specific container. The problem is the one you spotted, though. Their bonuses had nothing to do with when they show up. The belt bonus came out of the junk drawer, hours before you'd own a conveyor, and the tin-cup bonus turned up in the loft, long after the tin stopped mattering. I've reworked them along the lines you suggested. Early keepsakes now help the tin, magnet and coins, the middle ones help your cousin and the table, and the late ones help the belt and your cousin, who is the real bottleneck at the end.
These changes will be in the next update. Thanks again, really glad you enjoyed it!