Claim 1: there was no unofficial foreign policy because the President directs foreign policy.
Response: While it is not nearly as unitary as stated, the principle does hold true. What is also true is that a great number of important people were kept out of the loop regarding the pursuit of investigations, this pursuit and actions to incentivize it worked counter to other established foreign policy which workers who were not informed of changes were still pursuing, and the flow of pursuing this policy was a highly unusual channel. The claim is that Trump's pursuit of investigations was for personal gain in the election and unconnected and at odds with national security interests. If that is true, the conduct is illegal regardless of whether he pursued the investigations with the consent and aid of the usual diplomatic apparatus or specifically apart from it.
Claim 2: just because there are debunked conspiracy theories, doesn't mean Ukraine didn't interfere in the election.
Response: no it doesn't. Also Japan could have. And Finland. Etc. There needs to be evidence to support an investigation, and not liking someone is not sufficient evidence. More pertinently, Trump's phone call specifically referred to the CrowdStrike server which is that exact debunked conspiracy theory, and I'm not aware of any other reason Trump specifically was concerned. Even more importantly, the payoff for Ukraine rested in a public CNN statement and not the underlying investigations themselves.
Claim 2: in asserting there was no evidence supporting investigations from the witnesses, they exceeded their expertise because they didn't have access to the underlying facts.
Response: I don't know if any testimony offered exceeded anyone's expertise. Stating where your impression comes from, e.g. underlying intelligence agency opinions, direct personal experience with the parties allegedly involved, etc. is totally fair game. Their testimony needs to be weighed against the testimony of those who provide support for such suspicions. And what is at issue here is what was known by Trump and his foreign policy apparatus at the time and what efforts they took to ensure credibility of such evidence before acting upon it. Thus, for example, Hunter Biden is not a useful witness. He has no awareness of what Trump knew. Problem is, no one with the proper awareness has offered any testimony supporting investigations or what Trump or Rudy or Pompeo etc. may have known that the other officials who testified clearly didn't. The reason for this is defying Congressional subpoena for both records and testimony. Overall, the argument rests on a basic logical error. Saying investigations are justified because you can't definitively prove they aren't is untenable as a defense in absence of suitable justification for which I have seen none.