Section 3 of the 20th Amendment states: “...if if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.”
From this, if Biden is disqualified as well, the normal succession train doesn't apply, so Pelosi would not be president. Instead, Congress would have to pass a law determining who would be only "acting" president until the "qualified" winner of the presidental election process was determined.
Also, since there are no special instructions for how "a President shall have qualified," the presumption is that the normal procedure is still intact - i.e. an election. So if Obama is disqualified, that would nominally leave McCain as the qualified winner of the election.
However, in such a case there would certainly be a suit by Hillary that she had to run against an unqualified candidate who illegally sucked up votes that would have gone to her, and which would have enabled her to beat McCain. So there's a good chance that the whole election would be disqualified as tainted by fraud, and thus be required to be run again.
In the mean time, since the "acting" president (either Biden or someone specified by new Congressional law) would be prevented from being the "qualified" president, he would not have full presidential status, since the Constitution explicitly states that he exists only in order to be replaced.
So there is an argument that the only thing such an "acting president" could do is preside of the determination of the next "qualified" president, and not implement or change anything on his or her own - including signing bills. That, in fact, bill-signing could only be done by a "qualifed" president.
This might be seen as especially relevent since Obama has had such an enormous influence over the creation and implementation of these various monster bills. So their very existence could be interpreted as tainted and therefore disqualified by his fraud. Which would mean they not only would not be signable by the "acting" president, they might also have to be struck down as illegitimate creations altogether.
Section 3 of the 20th does not apply, since Congress certified the elections of Obama and Biden. That is the “qualifying” that was to be done under that clause.
An ineligible President cannot be made eligible, though, so resort must be made, to Article II, Section 1, Clause 6, regarding Constitutional disability of a President, and that is in turn further spelled out by the 25th Amendment, dealing with potential ambiguities of succession in Article II, Section 1.
Therefore, it’s clearly Biden. There has been no effort to incriminate him as having been somehow complicit in any attempted fraud on the part of Obama, nor has there been any such effort regarding any attempted fraud on the part of the DNC.
There can’t be such an effort regarding the former, until there has been sufficient, legal proof of fraud having occurred. Regarding the latter, the DNC, there is sufficient evidence to cause legal and political problems for Pelosi, should Obama ever be found ineligible for office. The party certifications have peculiar variances that more than just imply prior knowledge of a problem with their candidate, as far as eligiblity.
So, one thing at a time. It’s not going to come down all in one fell swoop, if it comes down at all.
Biden, it is, according to the Constitution. If, for some reason, Biden himself is found to have a Constitutional disability under Article II, Section 1, Clause 6, succession as dictated by the 25th leads then to Pelosi. But, that’s a separate matter not pursued to date, because it can’t be, yet.
If that point is reached, Pelosi will have big problems, and assuming Robert Byrd is still holding office and not disabled himself, it would be him.
That’s a whole string of what-ifs, though. The Constitution says Biden, and anything else would come after, according to separate actions and procedures.
But Obama is not the president-elect, he's the president. Anything subsequent to January 20th doesn't change that. So it isn't the 20th Amendment but the 25th Amendment that applies, specifically the first clause: In case of the removal of the President from office or of his death or resignation, the Vice President shall become President.
If Obama is found to be ineligible then he will be removed or impeached, in any event forced from office. But the actions of the Electoral College are not nullified or the inauguration voided.