Your timeline is very plausible. Of course, much would depend on how the alternative presidents and vice-presidents performed, or were perceived to have performed, in office. Not to try to run your timeline for you, but it would be interesting to speculate on such questions as what Humphrey and Muskie would have done about Vietnam (and Reagan and Schweiker, for that matter, if the war was still going when they took office). Or what Reagan and Schweiker would have done about the hostage crisis, provided you think there still would have been one with them in office.
I was hoping to flesh this out further in a Wakie-style timeline, but since it'll be awhile till I will even have time to do that (if ever), I might as well run through a discussion of some of the possibilities right now.
Anytime you're changing something major in history (such as presidents), there will be other changes that will be hard to foresee. Unless there is some direct cause for an event to not have happened, I'll assume that an event or something similar would have occurred. For instance, my timeline assumes that Ted Kennedy still has his Chappaquiddick incident, that Bentsen is elected to the Senate in 1988, and that George Wallace is still shot.
Hubert Humphrey's AdministrationI envision Humphrey winning a narrow electoral college victory but losing the national popular vote. The defeat spells the end of Richard Nixon, who withdraws into a haze of depression of bitterness (although he does receive an ambassadorship under Ronald Reagan).
I would expect that in line with the late '68 "Halloween peace talks," there would have been renewed efforts at a tentative "peace" deal that would enable troops to be pulled out. Whether Humphrey would actually have pulled the troops out is a legitimate question. We know now that Humphrey was privately very skeptical of the war and sympathetic to a pullout but publicly backed White House policy in order to prevent repercussions from Johnson. However, a President Humphrey may well have felt constrained by the political options and dithered on a withdrawal. I believe he would however have pulled troops out by 1970. South Vietnam would likely have fallen within a year or two and the issue with prove divisive at home and major issue with conservatives.
School desegregation would likely have proceeded much as in real life (possibly somewhat sooner, probably somewhat more violently); bussing would probably have been implemented as well. In fact, by and large, I would expect that a Humphrey administration would have a domestic record not all that unlike the Nixon administration.
As for China, Humphrey was an old cold warrior, but it is likely that by the 1970s, whoever was US president would have had to start opening doors to China. I don't think Humphrey could have afforded something as visible as Nixon's surprise trip (especially in the aftermath of an embarrassing Vietnam withdrawal), but backchannel communications and some thaw would have been quite possible.
Another interesting question: would the Electoral College still be with us? The institution came closer to being abolished in 1969 than it ever has before or since. Given close elections in 1948, 1960 and 1968, plus the threat of no electoral majority in '68, the House of Reps actually overwhelmingly approved an amendment to replace the EC with a national popular vote (with runoffs authorized if no one received more than 40% of the vote). Polls even showed it passing the requisite number of state legislatures and its Senate prospects appeared good too. A Southern filibuster, however, combined with opposition from many conservative Republicans persuaded enough wavering senators to vote against, however, dooming it.
Would the Republicans have opposed it unanimously if it had cost them the election in '68? For the sake of simplicity, I'll assume the institution would have remained, but it's something to consider.
Going into the 1972 elections, Humphrey's approval ratings hovered in the low-to-mid '40s. His administration had amassed a fairly impressive domestic record, but his withdrawal from Vietnam coupled with detente policies towards the Soviets and the Chinese AND the economic woes of 1970 would have made his reelection iffy to begin with. The fact that the right (Nixon and Wallace) won a combined 56% of the vote in 1968 illustrates what a hole he was in. Also, the country was coming off of 12 years of Democratic rule; a change in power was likely.
The 1972 Republican nomination campaign would have been a pitched battle between Ronald Reagan and the moderates in the party. Nelson Rockefeller would have run again, but his campaign would have been steamrolled by the Reagan campaign. Reagan's "radical" reputation would have encouraged the Humphrey/Muskie campaign, as early polls would show the incumbents winning comfortably. But despite doubts about his electability, Reagan would win the nomination and select a moderate-to-liberal Republican (such as Schweiker) as his running mate.
Assuming George Wallace still withdrew from the race (having been assassinated), I would assume a straight Humphrey vs. Reagan race would result in a narrow Reagan win.
