Just as a matter of record (or "FWIW" since this is just what I've observed from weekly trips on the LL)
The LL has been operating on the SW side of the ROW (the future GLX slots) from roughly Washington St in Somerville to Ball Sq (where it can't be moved until the Broadway Bridge closes and its abutments are "unpinched") I'd say it has been this way since late December '18 or early January '19
Midday trips seem to be a single track operation (entirely operating on the "inbound" track), which puts the trains even farther from the most intense construction through Somerville, and the LL-side retaining wall work at College Ave.
I would not be surprised to see midday single tracking go on this way until the demolition at the Hohman's site (Gilman Sq) is done. When demoltion at Hohman's is complete, the demolition work will move two GLX stops out to the 3 buildings at Balls Sq (do you think the single track ops would move to the outbound track at that point to stay a bit further away from the Ball Sq demo work?)
In Somerville, they are doing a lot of drainage work under the future/final LL, and in many places are making the width by excavating back to the "original" driven sheet pile wall. Running in this location allows them to have jersey barriers between the CR and the ongoing ROW construction and upcoming demolition of the Hohman's building in Gilman Sq.
Out beyond Ball Sq, the tracks shift back to either their final location or something of a "down the middle" (thanks to the "Construction Phase 1" work done in Tufts' neighborhood), which puts them away from the up coming demolition of 3 buildings on the future Ball Sq GLX station.
At College Ave, they continue to move slowly in building the retaining wall on the future LL side.
"Trying to solve congestion by making roadways wider is like trying to solve obesity by buying bigger pants."--Charles Marohn