I think Gabby was killed on the 27th - MOO, and here's why.
Like the posts before me, this too is speculation only:
I think something happened on the way to Yellowstone. The Victor, Idaho merchant stated that they told her on the 25th or 26th that they were headed to YSNP and she advised them to enter by the west entrance.
Business owner says Gabby Petito and boyfriend made stop in Victor after visiting Wyoming - East Idaho News This entrance would be via West Yellowstone, MT.
Google Maps
MOO and speculation: They could have been in Victor, ID on either the 25th or the 26th and this speculative timeline might still work since Victor, ID is only 1.5 hrs from Spread Creek per Google maps.
Something happened between them after they left the shop in Victor, ID that made them turn back. Don't know if it was right away, or further along the route to YS, but Jessica Schultz saw the van back at Spread Creek on the evening of the 26th.
Gabby Petito case: What one witness saw near the Tetons may help FBI unravel mystery
Again with the MOO and speculation and IMO caveats, whatever it was that occurred between them may have still been festering on the 27th - through their time at Merry Piglets TexMex.
Speculative conclusion: whatever happened to Gabby happened on the 27th or the latest the 28th. By the 29th he was already trying to discard evidence, cover his tracks, set up alibis, and hitch rides back. MOO, JMO, IMO, and all other acronyms to convey this is not being stated as fact.