Spectre allows reading of all memory in a process's address space. In an app it's assumed that you already have access to the full address space, but this is not true in the browser, where usually a single process will contain data from multiple web domains.
Chrome has always been returning microsecond resolution for these queries rather than nanosecond resolution. In discussion with the GLitch researchers, it seems likely that this reduction in precision is sufficient – and since no WebGL developer ever complained about low resolution of Chrome's timer queries, there's no need to make any changes to the precision.
I don't know exactly. Site Isolation is being rolled out now in Chrome. It's a large enough feature that if you watch the Chromium and Chrome blogs you'll definitely see the updates.
I don't know, but all browsers have a vested interest in turning SharedArrayBuffer back on, in particular.