"If it's what you say I love it." -Don Jr. in response to an email that was prefaced with, "as part of the Russian government's ONGOING EFFORT TO HELP YOUR FATHER..." And still no questions from the GOP members of the committee. How is an email record that different from tapes? The fact that Jr. didn't say, "If it's what you say, I love it, but wait, why if Russia trying to help my dad? That seems odd." is enough evidence that this wasn't an isolated incident. If that was the first he heard of Russia's effort to elect Trump and he didn't react, then he's literally the stupidest person in public life (since the language was so obviously inserted to use against him if necessary). The other explanation is that it was just routine at that point. Either way the evidence is pretty plain, so maybe even tapes of Trump saying things that even the tin foiliest hatted person in the world couldn't have dreamed up will never be enough....there are probably no audio recordings of Trump et al admitting plainfully their coordination with Russians...
So if GOP heel-dragging is what's gumming up the investigation, what do you see happening if Democrats take over the House in November? (I'm not sure how likely the Senate takeover is. A lot can happen in eleven months but taking the House should be sufficient, no?)
Probably most will refuse. That's when Congress has to sue. This also happened in Watergate. The committee subpoenaed the tapes, and Nixon refused to hand them over (claiming executive privilege). Eventually, the Supreme Court forced the administration to comply. One major difference here is that the Court is occupied by GOP sycophants who have no trouble violating their own stated principles in service of the GOP's agenda. Like with Watergate, getting at the truth will depend entirely on enough Republicans turning on him. If the Grassley/Graham questioning of Fusion GPS is any indication, we have a long way to go. As to your second question, my gut is to not dignify it with an answer. However, it should be pointed out that it is explicitly stated in the Constitution that a member of Congress cannot be arrested when traveling to/from Congress or when in session.