What are viable scenarios that Nancy Pelosi becomes President?

That's not how it works. That's not how any of it works. And the stupidity of Americans about their own Constitution pains me.

If the President is removed from office the Veep automatically becomes president (Amendment 25, section 1). He then nominates a new Vice-President, who takes office on being confirmed by both chambers of Congress (Amendment 25 section 2).

So if the Vice President is removed from office, the President nominates a new Vice-President, etc. (section 2, again).

The only way Pelosi becomes President is if both Trump and Pence are removed from office simultaneously. Death could do that (oh, sweet meteor, deliver us from evil), but for impeachment to do so would require the Republican controlled Senate to impeach Trump and Pence simultaneously - it would require Republican agreement to hand over the White House to the other party, and to someone they loathe.

What do you think the odds of that are?

Assuming there are enough Republicans in the Senate who would vote for conviction that removal of either Trump or Pence is possible, the Republican's move in the case of dual impeachment is simple: hold the trials one at a time, and give sufficient time in-between to appoint a new VP.

The Democrats' option then is to refuse to join the Senate in approving the new Veep, leaving the office vacant. At which point Republicans who might have been willing to convict the second person tried are so infuriated at the naked power grab that they either vote against conviction or simply refuse to hold the trial until the Democrats agree to approve the Veep selection.

Either the Dems give in because they're taking a political shellacking in the press leading up to the election period, or they hang tough and there is a stalemate that leaves the Vice Presidency vacant until January 20, 2021.

Either way, Pelosi does not become President. Period. Unless you can control a meteor.