It wasn't tampered with. The speedometer is off because the transmission was swapped to another gear ratio transmission. It's common.
It's impossible for the timing belt to cause metal shavings in the oil like you're describing.
You never told us what you meant by "timing slip". You never said it was fixed - just that you had it looked at, they said it was fine to drive, and you drove 400 miles. That doesn't sound like a problematic timing slip. It doesn't matter what happened, we know it's unrelated because when a compromised timing belt damages the engine it can't in anyway introduce metal to the oil. So they are unrelated.
Also, if it slipped enough to cause damage - it would have needed repaired the moment it happened.
If timing belts slip on an interference engine (which yours is), the pistons bend the valves, rendering the car undrivable, or it drives terribly, or no damage at all. It wasn't undrivable and you didn't say any severe performance issues. So there was no damage.
The moment the belt "slips" (ambiguity notwithstading) it's either damaged or not. It can't cause issues later.