Oh, I didn't say anything about impeaching Joe. The issue of corruption in the Ukraine and steps that were taken/not taken before and after Trump's involvement are central to establishing context for any discussions recorded, overheard, and testified too. Without question, the roles of Biden and Hunter and Burisma can be better understood by having them testify.
The claim is Trump withheld foreign aid for Ukraine to convince them to investigate his political rival. That it the claim and what is being investigated. What Biden may or may not have done is irrelevant and not material to the case because it does not make the quid pro quo request or actions suddenly legal. Does not mean it is not an issue in general, just not for this case. Law does not work that way. Biden's role is only relevant with respect to Biden and not to this impeachment hearing party. You have to understand the law, Biden is not part of this so what he did or did not do does not suddenly make an illegal action legal.
If you do understand the law, then you know that there is no legal or constitutional barrier to the Senate calling Biden, Hunter, and the WhistleSpy if they so desire. The fact that you do not think they are relevant to the issue at hand is not dispositive. Further, an impeachment proceedings is not a criminal proceeding and is not bound by the rules of criminal procedure, even if I were to accept the assertion that their testimony would not be relevant, which I do not.
No, this is totally wrong. Trump's actions become questionable only to the extent he lacked a good faith basis to believe there was reasonable cause to have Biden's outrageous conduct investigated, free of Biden's threat of an aid cutoff. Otherwise, your rule is a president can never have a political rival investigated, no matter how outrageous his conduct, which is obviously not the law. Central to that determination would be a searching examination of the Bidens' conduct, so that we would know if Trump acted on reasonable suspicion or merely political malice. The dems are attempting to skip this crucial step, which renders their entire proceeding a farce.
The other aspect of this that is important is what started it. The dems are pretending Trump leaned on the Ukraine to "dig up dirt" on Biden. But in fact, Biden himself put the incident into the public domain. Having apparently exhausted the Corn Pop legend, Biden foolishly bragged about using US aid to intimidate the Ukraine into firing a troublesome prosecutor. The fact that he didn't see this as troublesome is worrying enough. You have to be a blind partisan to not see that Biden's actions constituted a massive conflict of interest at best, and outright corruption at worst.