It sounds like it, in theory. Interesting that some folks didn't want it, and I'm not sure a pardon (especially a blanket one) can be refused. It's bad precedent on Biden's part, though a lot of the promised/threatened prosecutions would also have been bad precedent.
Other than Ford pardoning Nixon, I can't recall one quite on this level. Usually, there's a ton of pardons that would either not have been politically popular, or are obvious quid-pro-quo (Trump was the master of this last one), but a preemptive one for a large group isn't one I think we've seen before.
Not at all - in fact the opposite. They have no immunity to testifying because they are not able to incriminate themselves when they have immunity from prosecution.
We need to get rid of the presidential pardon system, or completely overhaul it. It's a clear abuse of power and it happens at the end of every presidency.