I see this a lot... and while I agree that there was a great level of stupidity in this whole thing from the Trump camp the notion that Jr. tweeting the emails was a stupid move is just not how I see it (I realise this is not exactly what you are stating, but this is something I have been wanting to address, so I apoligize for going a bit off the topic your statement is discussiong- i.e. how mindnumbling careless they seem to have been in their contacts with russia). We have to always remember their main concern right now has to be the number of investigations that are taking place. As such, releasing the emails was his ONLY line of action to try and deflect that threat. By releasing the emails the attempt is to write the narrative and do damage control, since they KNOW they where going to surface as part of the investigation. Sure, they could have denied the NY Times or the Post releasing the emails by saying they are fake, but they know well that doesn´t fly with the investigation. In this context its better to face the music and confess to the meeting, and try to advert repercussions. They are of course hoping leaks end here, which I highly doubt.
But the actual tweeting of the emails is far from dumb. They are pretty cunning in doing so actually. It helps them build the best possible counter deffense (the dems are the ones to blame) while stating their desire to be transparent (to condition those supporters with low critical thinking skills willing buy it). In short, they can attempt to frame their wrongdoing on their terms and build on that narrative while deflecting blame.