I was thinking about what you said still, Darby.
In theory, you’d be correct at first.
But in reality, I don’t think it makes sense by the end… I am going walk out what i think is the reverse of your points. “devil’s advocate type ish”
You mentioned Pamela and her disk?/computer being a weak link.
I agree, that’s a safer bet, but… finding John doesn’t guarantee getting the machine, and finding the machine doesn’t guarantee getting John.
To catch him, a traveler has to know the exact time and… precise location.
That is a huge ask for any time traveler.
Plus wouldn’t people notice an huge influx of weird people flooding out one specific area over time?
If thousands of future travelers have ‘forever’ to go to that one moment, or good lord how many other, it becomes incredibly congested.
They risk getting arrested for at least loitering, they risk messing up history, or… being interrogated by the government if the police call in back~up/higher~ups after said strange interrogation.
Imagine a past government getting their hands on future time units to back engineer them, all because a (t) traveler got sloppy trying to find Titor.
It gets worse if John was right about the multi-world theory.
Travelers would just be jumping into random parallel timelines where John did things differently.
Lets assume they kept logs, how do logs even help then, they have so more work to do if they thought it was a single worldline interpretation & found out… just how late?
You’d be chasing a trail that doesn’t even exist in that worldline, or… diverging ones in others.
If he was there, what if he would have went left instead of right at any moment & vice versa.
Going after Pam seems safest, but John surely would have warned her about these kinds of time~traveling scalpers & safe~word"s"/song"s" helps.
Note: I would also like to mention I do not personally think she is a weak link.
“especially since she has a time traveling friend” 
At this point I do not think we will ever know the song for sure, even knowing that… only gets you so far.
It seems like a ton of trouble & unreasonable risk for minimal reward.
Why would any time traveler even waste time to rob a store when they’re already rich?
“in a manner of speaking”
~QS