Drunken Irishman, this is a well-reasoned post, and deserves a healthy k&r which I am glad to give. I agree that even beginning a formal impeachment inquiry immediately would be premature for many of the reasons you outline.
However, I have a different take than what you say here:
That's a tight rope to walk. I believe hearings should continue - as we've seen - but if those subpoenaed to testify continue to refuse to, and the White House stonewalls, and Mueller refuses to say anything beyond what's in his report, the optics aren't going to be there for the Democrats and once you go down the path to impeachment, there's no coming back or you've just handed Trump a monumental victory.
I agree that all the disparate Congressional Committee hearings do indeed need to continue. And I will bet good money that any subpoenaed witness that 45 can reach will be pressured to refuse. Barr will continue to obstruct and hide documents. Each such instance can and should build the case for impeachment, and some time in late June or perhaps July, that case will become overwhelming to even the fence-sitters. Such a time frame will also give Democratic Presidential candidates the chance to articulate the need for impeachment on the debate stage. Given the crimes and evidence we already know, if investigations, documents, and witnesses have not all moved forward in significant ways by late July, Pelosi (or any Speaker worth her salt) ought to be able to convene an impeachment inquiry vote with fewer than ten Democrats dissenting and hopefully three or more Republicans in favor.
I think the alternative of allowing both the past crimes and the ongoing obstruction remaining unaddressed by impeachment through the end of summer or beyond is an absolute dead end for Democrats, and hope that Pelosi can see that.
-app