Thank you for confirming the 43:23 closing time for Brest (614km) in 2015. I note that that's 14.15kph (and my exact 14kph estimate reflects that our 2019 version is 12km less overall).
Those average speeds will give you incorrect intermediate control times between Paris and Brest and (to a greater extent) between Brest and Paris.
I'm sure you're 'correct' if 'incorrect' means more than 20 minutes 'wrong'. When you say 'incorrect' I hear an echo of @QuixoticGeek's post about UTC not being the same as GMT because of their different treatment of leap seconds.
On the way back the protocol suggested in the 2007 link in the RUSA thread seems designed to make the control close timings back to Mortagne a tad more 'relaxed' recognising that riders may bump against the closing times yet be able to raise their tempo for the final 100km. Entirely sensible.
I'll just put my (90 hour 1800 start) closing times up here (14kph out, 13kph back) so I, you (et al) can see how incorrect I was, in August.
Rambouillet 18:00
Mortagne-au-Perche 02:21 MON
Villaines-la-Juhel 09:25 MON
Fougères 15:45 MON
Tinténiac 19:38 MON
Loudéac 01:41 TUE
Carhaix-Plouguer 07:07 TUE
Brest 13:28 TUE
Carhaix-Plouguer 19:51 TUE
Loudéac 02:29 WED
Tinténiac 09:06 WED
Fougères 13:16 WED
Villaines-la-Juhel 20:06 WED
Mortagne-au-Perche 02:34 THU
Dreux 08:32 THU
Rambouillet 11:55 THU
"Consider a hypothetical BRM 200 (8am start) that was over distance"
Aware of the 'overdistance' massaging that is required.
But the PBP massaging is not really to do with that.