If the SNCF is running replacement buses, it will run them from the train station, not the bus station;

Gare Routiere and Gare Centre are close to each other. That's good.

And if I go with TER (leaning this way as it's only 15 minutes longer and less hassle) I can buy them in Paris a few days before.

Every gare routière and gare centrale are within close proximity to each other, for obvious reasons. What kerouac is trying to tell you is that if your train is canceled and replaced by a bus, the bus will come to you at the train station. If you go to the gare routière in that instance you may get confused and miss your bus.

St Cirq, Kerouac,
It makes a lot of sense the way you explain it.
But why does the SNCF web site show it as Gare Routiere. Just to confuse matters I guess.

vbahn.de also shows 800 meters from station to Gare Routiere -maybe even roads around station are torn up? Since you'll be there day before simply go to train station and ask.

If the SNCF site shows the connecting bus leaving from the bus station, then it is almost certainly correct albeit a bit strange -- but sometimes there is no place for a bus to wait in front of certain train stations, so that might explain it.

And, since no composteurs in bus station just do it when there is a 10-minute change time back to train.

Often replacement buses run free of charge anyway. I recently went to Auvers-sur-Oise (the Van Gogh place), and the train line was closed for works on the tracks. The replacement bus ran with absolutely no verification of anything. Believe it or not, the SNCF does not like to add additional annoyances to inconveniences.

The same thing has happened to us twice in the past year. No train running? Hop on the bus, and that's all. No showing of tickets, no verifications, just get on and we'll get you there. SNCF also paid 190€ for my BFF to take a taxi from Agen to Le Buisson when she was mistakenly told in Bordeaux that she was on the right train to Le Buisson, but wasn't (there were two trains leaving at almost the same time and she didn't know to match the actual train number on her ticket with the one on the station screen). It would have been very easy to claim it was her mistake, but SNCF took her word that the conductor assured her she was on the right train, and when it turned out she wasn't, they put her in a taxi. We've gotten free taxis from Périgueux to Les Eyzies twice when our trains were late because of track work. I have no quibble with SNCF customer service.