Because I can't help myself, I made a map of the current service.
View attachment June 2022 System Map during shutdown.png
To the question of why GLX is terminating at Lechmere -- I wonder if they are just single tracking it between Lechmere and Union and only want to use 1 train for it -- leave the rest to bolster service between Copley and Government Center where, as you can see, the Green Line will be asked to absorb a large number of Orange Line passengers.
View attachment June 2022 System Map during shutdown.png
To the question of why GLX is terminating at Lechmere -- I wonder if they are just single tracking it between Lechmere and Union and only want to use 1 train for it -- leave the rest to bolster service between Copley and Government Center where, as you can see, the Green Line will be asked to absorb a large number of Orange Line passengers.