I forget, why exactly was 1934 important? Was that when Deido constructed Souboutei?
Also, the whole time-travel things seems a bit wibbly-wobbly. If time is so distorted in the tunnels, how do they know that they'll arrive at the same time going back into Souboutei? We see that all the tunnels lead to 1934, so if they travel out from 1934, doesn't that mean they could end up in any time? Even if it's "roughly" the same time, a minute or two could be the difference between life and death for the others.
And why do they even need to pop out and go back in? Can't they find a "junction" within the tunnel like in the diagram? How do they even know going back in through a tunnel will land them directly in front of the atlier? Clearly, everybody entering Souboutei in the present didn't get sent to the atlier OR get sent back to 1934 (at least not directly; they met Zanka and and Makoto, but the central building is in its modern configuration). Maybe it's like a M:tA "reality consensus" sort of thing, since Souboutei was being televised on the outside?
Side note, technically a previous translation had it right: Seiichi and Makoto would be Takoha's uncle and aunt, not the other way around.