Great questions.
As to 18 USC § 1503, the general rule there is that it must involve an existing judicial proceeding: a grand jury (or current trial in front of a petit jury or judge) must be involved. As we learn from US v Aguilar:
The § 1505 provision was discussed in a number of cases. As we learn from US v Price, the crime of obstruction of justice under 1505
There’s little doubt that there was a “proceeding.” The courts are generally inclined to hold that agency investigative activities are “proceedings” within the meaning of § 1505. US v. Browning, Inc.
And I don’t think there’s much doubt Trump was aware there was a proceeding.
So as you suggest, the key element relates to “threat.”
A threat is an expression of intention to illegally inflict evil, injury, or damage; a purpose or intention to work injury to the person, property, or rights of another without right of law. US v. Poindexter found that the word “corruptly” was too ill-defined to be constitutionally applied against Admiral Pointdexter for his role in the Iran-Contra affair, and “threat” must be a statement that shows an intention to illegally injure a person, property, or rights.
With that in mind, context is everything. A literal parsing of words cannot transform a threat into nullity; the “nice place you got here; shame if anything happened to it,” is a threat even though the words taken literally are not. But “If you don’t tell us what happened, I’ll arrest you for murder,” may or not be a threat. If the speaker has the legal right to arrest and probable cause to do so, the intention is not to illegally impair the target’s rights. But if the speaker is contemplating a false arrest, then it’s an illegal threat.
I haven’t heard anything in the discussion of Trump’s words that rise to this level. What’s been reported thus far is merely Trump saying (in February) that he hoped Comey would conclude the investigation. I’m open to education on this point, but there does not seem to be any allegation that Trump said anything that would constitute a threat.