Digging a tunnel might seem like a smart idea, but his storage is still only the size of a large warehouse. Even that would seem limited against several thousand tons of mountain. Not to mention he needs to dump that debris somewhere at some point, and I don’t even want to think about factoring in tunnel integrity and ventilation in the equation
Ventilation isn't really an issue when he's excavating large chunks of the mountain all at once; he's not doing a lot of physical exertion and consuming what limited oxygen is located there, then breathing air concentrated with carbon dioxide to the extent of CO2 poisoning while exerting himself. The mass removal of stone followed by its ejection outside the tunnel would also provide more than sufficient pull (via vacuum collapse) and push (via materialization displacement) of further air into the tunnel.
If you mean within the tunnel afterwards, then the draft from one end of the tunnel through to the other end would suffice. The Pausilippo is 4,800 feet long, 25 feet wide, and 30 feet tall, dug entirely by hand, at a rate of 5-7m per day, from both ends. Our boy could easily do that in an hour, if that (emptying his inventory entirely would enable him to dig much further before having to stop, and if he made it much smaller, enough to suffice for two carriages side by side and a bit more for foot traffic, he could delve much, MUCH further in one go). The average mountain varies in width from 3000 feet (reminder for those of us unfamiliar with the conversion, 1 mile is 5280 feet) to 5 miles wide, while large mountain ranges that are many mountains wide vary from 50 to 300 miles in width, depending on where you're measuring at, so it'd only take him a couple days in the worst case scenario of having to go through the wide belt of a range; especially if the Red Cliff team act as porters, ferrying him in and out of the tunnel in shifts.
