By diesel, do you mean a straight diesel, or a hybrid? The 73 under normal circumstances should operate only with the hybrid "Xcelsior" New Flyers (the ones with the kind of streamlined look), numbers 1925 or higher. Route 554 to Newton Corner via Waltham runs out of Albany St garage, which does use older straight diesels. The 554's usually lay over on Trapelo Road, while the 73's use Church Street.
I'll try to post back here when any of the overhead I encounter starts to disappear. So far, nothing's been touched on the portion of the 73 within Belmont, but I suspect that will be some of the last wire to come down. With construction projects on Mt Auburn St (at least the whole Watertown portion, and possibly up into Cambridge, too, Huron Ave (west of Fresh Pond Parkway), Belmont St (between Belmont line and Mt Auburn), and Mass Ave, I expect any of those areas to see the wires cut sooner. I would assume most work will happen at night.
The new schedules for these routes appear to need some tweaking. It appears that at rush hour, the operator's layover/recovery happens at Watertown or Waverley, and the buses do an unload/load/go at the Harvard upper tunnel. No time to hold for a schedule adjustment at Harvard, but they should only be working off any inbound delay minutes. They do still have to transit through Harvard Sq on the surface to get back to Mt Auburn St.
Off-peak (especially midday), the pattern reverses, and the longer layover shifts to Harvard. Buses arrive into the upper level, unload, then drop back through the Square to Bennett St alley to wait for outbound departure. At that time, they go back into the upper level tunnel, load, and have to loop back through the Square again to head out the route.
I've only watched a couple middays on Pantograph App (
https://www.pantographapp.com/newengland/routes) but it seems as the midday period goes on, the outbound buses fall further and further behind schedule. Being 4-8 minutes off is normal, but I've caught much higher at times. I would imagine at the next schedule rating they're going to need to add more time to the off-peak schedules to cope with traffic in Harvard Square.