I've seen a lot of learned commentators on both sides of the impeachment debate arguing that the House *must* follow certain procedures (or not), or that the president *must* cooperate in the following ways (or not). What almost no one ever does is quote the relevant constitutional text, which is ridiculously sparse: "The House of Representatives shall chuse their Speaker and other Officers; and shall have the sole Power of Impeachment." There is nothing about what procedures the House must or may use, nor is there any indication to what extent the president and other executive branch officials are required to cooperate.
What we do have is historical literature on what impeachment was thought to be in 1789, analogies to other civil, quasi-criminal, and criminal proceedings, past practice by both the executive branch and the House, and any relevant Supreme Court precedents on related matters (even though most impeachment disputes will not be justiciable, all parties still have to fulfill their constitutional obligations, which judicial precedent might speak to.)
But how, for example, would one weigh executive refusal to cooperate if, for example, historical practice, the "best" analogy, and SCOTUS precedent all provide different answers? Color me skeptical that there will generally be firm answers to the questions raised by impeachment other than however the political process sorts things out.