This is my opinion and in no way I am bashing your idea (I actually like how many people have ideas to make the player assume the role of Courier and I would love mods of all of them
).
I don't understand why it would be more lore friendly having a brother of James travel to the Mojave without the train (since the DC train station is in lockdown mode since the bombs fell), have time to do all of the things that the courier did before the start of Fallout New Vegas (like Tricky said, founding the Divide and visiting New Reno but there is also in game references of the Courier being in Utah, Circle Junction, Fort Abandon, and that the courier also went on Brahmin Rides at the Big Circle which is a route between several settlements including Broken Hills, New Reno, Redding, the Den, Modoc, Vault City and sometimes Gecko and to finish there is a dialog in Fallout New Vegas that implies that the Courier impregnated (or believes he impregnated) a woman in Montana around 18 years ago) and then still has time to return to the Capital Wasteland with the Platinum Chip and give it to the Lone Wanderer. Also the Platinum Chip that the Courier carries before the start of Fallout New Vegas was picked up at Primm (as said to the player by Johnson Nash, the Primm supervisor of the Mojave Express) so how can our character have the Platinum Chip being given by James brother and then the Fallout New Vegas game tell you that your character actually got the Platinum Chip from Primm when the first courier (Ulysses) refuses to accept the job so our character does it (which is an essential part of the whole plot of the Lonesome Road and Ulysses story and spans from the vanilla Fallout New Vegas and continues through all of the DLC's until it ends on the last one, Lonesome Road).
Again I would love to see your suggestion being built as a TTW mod
I love alternate ways of placing the player as both Lone Wanderer and Courier. I just don't think it is more lore friendly that what TTW has at the moment (for all the reasons I mentioned before).
And welcome around the Site and the TTW community
.