At the time of asking this question, President Trump has been impeached by the US House, but no trial date set by the US Senate. However, if Trump were to grant blanket pardons to all those who participated in the US Capitol riot today (Jan 15 2020) and he himself was convicted of inciting the same insurrection, would the pardon stand or would it be null and void?
The pardons would stand and continue to be valid.
There is a minority view that the "except in cases of impeachment" language in the pardon clause of the U.S. Constitution deprives a President of the pardon power after impeachment until there is a U.S. Senate non-conviction.
But the majority view is that this clause merely states that the loss of political office and prohibition on seeking future political office resulting from a U.S. Senate conviction in an impeachment trial cannot be removed via the pardon power. In the majority view, a President has all of the powers and authority of the office, including the pardon power, until the moment of a U.S. Senate conviction following a impeachment by the U.S. House (or the end of his term of office due to resignation or expiration of the President's term of office).
Neither view, of course, has ever been resolved authoritatively in the courts because it has never come up before historically.
Also, a group pardon would not absolve the President himself of criminal liability. The majority view (again never tested because no President has ever attempted to do so) is that a President may not pardon himself at all. But all other persons who benefit from the pardon would be relieved of criminal liability as a result.
If Trump were to be convicted by the Senate, either before or after he leaves office, that would not invalidate any pardons he might have previously issued, including for participation in the events of 6 Jan 2020. The president, while in office, can pardon any crime under US Federal law (which includes local DC law, because DC is a federal district). Such pardons are not reviewable or cancelable by Congress, nor by a future President. In theory a Constitutional Amendment might undo them, but that has never been done.
(This answer takes no position on whether an impeachment trial could continue after Trump's term of office ends, as that question appears to have no clearcut answer at this time, and is not part of the question asked.)
However such a pardon would not remove the restriction in section 3 of the 14th amendment that prevents anyone who has engaged in insurrection or rebellion after having sworn to uphold the constitution from holding office. Only Congress can remove that bar. But it probably would not apply to most of those who entered the Capitol, as most would not have previously sworn to support the US Constitution.
No. Presidential Pardons are not reviewable as part of impeachment process (unless a pardon was the cause for an impeachment article, even then, it is not recinded following a conviction.). A conviction on impeachment can only server to remove the President (or other officer) from office and can have no other punishment attached. The purpose is to remove from office so one could stand trial for a criminal act. A resolution barring future service in office may be passed after conviction with a 51% majority vote.
It is highly unlikely Trump will be convicted at trial (given that Impeachment process stops when the officer is removed from office by other means) and thus the likelyhood of a trial beginning, let alone concluding is not likely to occur.