I believe that Benjamin Franklin said something to the effect that impeachment was the solution to removing bad leaders by means other than assassination (though it might have been said of frequent elections, which was fairly new at the time of the Constitution.). Either way, in America, there is a general unwritten rule that you cannot prosecute the sitting President for any crime, so the solution is to unseat him, which is the only thing impeachment does (the only punishments are immediate loss of office held, and possible banning from running for federal office. No jail time or arrestable crime). If the impeached president doesn't surrender the office, legally the likely course is the secret service arrests him for trespassing on the White House property, since he's now a citizen with no business being there.
The Ford pardon was highly controversial at the time because Ford was such an unusual figure in American History (he was not elected Vice President, but appointed by the Senate after Agnew resigned for a completely different matter than Nixon. Without a sitting Veep, the Democrat Speaker of the House would be next in line for Presidency. The Speaker believed he would lose Republican support for Impeachment as it would look like he was attempting a coup of the office... so he quietly proposed to Nixon that if Nixon nominated then-Senate Minority leader Ford to be Vice President, he would lobby Democratic senators to confirm Ford. Ford's own memoirs suggest that at the time of his nomination, he understood that he was all but certain to be president soon after he became Veep.
Nixon's decision to resign followed a count of votes in the House Judiciary Committee that revealed that they had support to move forward with articles of Impeachment, having lost key Republican defenders in both houses that even the Senate was no longer safe (he didn't have a 1/3rd minority support there). Resignation of office halts the Impeachment process since again, it only is capable of removing someone from office. If they resign, the process is moot.
I'm not sure if there is any conclusive evidence to suggest that Nixon asked Ford for the pardon, but even if he had, there was no way Nixon could have forced Ford to pardon him. Only the President can issue pardons, and there's still much debate if the President can self pardon at all. So Ford could only pardon his "boss" Nixon once Nixon was not his "boss" and lacked the authority to issue the Pardon prior to Nixon's resignation, when the request could be made.
While Ford did Pardon Nixon and it was highly controversial at the time, Ford had maintained that his decision to do so was that the Watergate scandal had been a rather trying time on the nation's morale and that a legal trial would do more harm to the nation than any good and he wished to avoid this. Ford figured that the best thing to do was to allow the nation to heal and move on from the "long national nightmare" that was Watergate. At the time, Ford's pardon of Nixon was deeply despised by the American public and was probably the only reason Ford was not re-elected (despite the nation's dislike of the Pardon, Cater's victory over Ford in the 76 election was very narrow (winning by less than 2 million votes nation wide and 297 to 240 in the Electoral college, a gap that could be recovered by Florida and Ohio if they had broke for Ford. In fact, West of Texas, Carter only won Hawaii, and and Ford's approval was rising in the weeks prior to election day.). Ford did recognize this, once lamenting that he would only be remembered for pardoning Nixon (not true, as he's also the only President to have an assassination attempted against him by a woman... twice as a matter of fact... and within weeks of each other... and not related at all. That's slightly better than the guy who died 30 days into his presidency. Also, Ford was the first president to be mocked on Saturday Night Live and as such, his reputation as not having good balance is still noted of him.). In a rare move, Ford also appeared before a Congressional committee to explain his decision to pardon Nixon.
While at the time he was criticized for the decision, many of those critics have reversed on the issue, and thought that ultimately, Ford was right on the reasoning for his pardon (that the nation definitely didn't need the criminal trial on top of the drama that was Nixon's presidency. Keep in mind, that prior to the scandal, Nixon was a very popular President, winning re-election in a landslide of 49 states (only Massachusetts and D.C. voted against him) and he would have possibly been considered one of the nations greatest Presidents had he not engaged in the scandal. Ironically, Nixon didn't need Watergate to win his re-election as much of his success in 72 was because the Democratic candidate was incredibly weak... and his running mate selection was a tragic mistake and his handling of that situation further weakened his support.