If you're referring to the "viaduct" (actually mostly retained embankment) across the erstwhile Mumps roundabout then I've not heard that story but I would think if that was the idea it would have been abandoned pretty quick once 3B got the go-ahead. They would have had to demolish it with a closure of several months before it could be linked to the at-grade 3B route. As it is they connected 3B to 3A with a point on the outbound track only, so trams could get on and off the central section for testing and training, then re-aligned the other track during a much shorter blockade at commissioning. At the Werneth end the divergence was on a slab that had fixings for both routes so they could remove one set of rails and install the other.
I'm pretty sure if it had all been given the go-ahead at the same time that the tunnel would just have been abandoned and the trams run through the town centre from day 1. As it was, by the time the tunnels were being converted they knew they were temporary - I think they did keep the track and they certainly tried to save as much cost as possible recognizing that it wouldn't have to last long. Most of the OLE could probably have been unbolted and taken away even if intended to be permanent.