We know George and Lorraine Mcfly meet in a 1955 without a Marty, because Lorraine meets George after he gets hit by Lorraine's father's car. So, we can safely say there was a 1955 timeline with zero Martys. We only see a single Marty in the original BTTFs visit to 1955, but we know from the sequels more Martys arrive during the same time period. BTTF Marty interferes with the original 1955 timeline. Are the BTTF 2 & 3 sequels Martys interfering with the original BTTF Marty 1955 timeline? Was there really a 1955 Enchantment Under The Sea Dance timeline with just one Marty Mcfly?
It all depends on which time travel theory you choose to adhere to. A popular theory is that of the "many-worlds" interpretation of quantum physics. Although it has yet to be validated by science, this interpretation has the distinct advantage of allowing breaches of causality while not necessarily generating paradoxes.
What this interpretation states is, basically, that each possible outcome of a given event actually takes place, but in a separate universe. For example, each time you order pizza, there are many different universes "created" from that point in which you order every possible combination of toppings.
In the second movie of the series, Doc explains a theory of time travelling that is very similar to the many-worlds interpretation but, perhaps, slightly less costly in universes. When he draws the timeline graph, Doc explains to Marty that it is possible to create an alternate reality by altering important events that happened in the past. This theory stems from Doc's observation of an alternate 1985 where Biff pretty much controls Hills Valley. We later learn that this was due to Biff himself, from 2015, travelling back to 1955 to give himself the almanac.
If we choose to subscribe to this theory, there are many different timelines in the BTTF franchise. Futurepedia lists eight of them, but I'll summarize them here for an easier read.
The original timeline, in which George got hit by Sam Baine's car while peeping on Lorraine. They fall in love, but George is forever bullied by Biff. This is the timeline with zero Martys in 1955.
Marty accidentaly travels to november 5th, 1955. Marty saves his father from Sam's car, taking his place in Lorraine's heart. From this event stems the new timeline, leading to George becoming a bestseller author after standing up to Biff. This timeline has only one Marty in 1955.
This timeline is created when Doc enlist Marty's help in changing the future (the 2015 from the timeline 2 point of view). This timeline's "past" until 2015 is identical to timeline 2's, and there is thus only one Marty in 1955.
Old Biff creates this timeline by stealing the DeLorean in timeline 3 to bring the almanac to his younger self in 1955. Old Biff arrives in the morning on november 12th, the same day Marty leaves this era. In this timeline, there is still only one Marty, but two DeLoreans.
Doc and Marty go back to 1985 and realize what Biff has done, creating a new timeline. Although timelines 4 and 5 are very similar, they have a different future starting from 1985 and are thus considered different timelines. In this timeline, there is only one Marty in 1955, as those events were unaffected by the time travel, but there are two Martys in 1985 (one being in an oversea boarding school).
To prevent the changes brought by Old Biff, Doc and Marty go back to 1955. They arrive on november 12th, meaning that the events from timeline 2's 1955 are still in progress. However, by preventing Biff from keeping the Almanac, the events from timelines 4 and 5 never take place, leading to a brighter future. As George still stands up to Biff, it is probably safe to assume that the future of timeline 6 is identical to that of timeline 2. In this timeline, there are two Martys in 1955, as well as three DeLoreans (the one Marty used, the one Old Biff used and the one the other Marty used).
This timeline is created by Doc going back to 1885 after the DeLorean was struck by lightning. This timeline is mostly identical to timeline 6, with the exception that, although there are still only two Martys in 1955, there are now four DeLoreans (the 3 from timeline 6 and the one in Delgado Mine).
This timeline is created when Marty travels to 1885 to save Doc's life. Up to 1985, this timeline is then basically identical to timeline 2. However, Marty refusing to race against Needles ensures that the events unfolding from that point will not lead to the 2015 we have seen in the movies. These events do not influence what happened in 1955, and there are thus still two Martys in 1955 in this timeline.
Of the 8 different timelines depicted in the movies, there are:
- One with zero Martys in 1955 (timeline 1)
- Four with only one Marty in 1955 (timelines 2 through 5)
- Three with two Martys in 1955 (timelines 6 through 8)
So, to answer your question, I would say that there is indeed a timeline with only one Marty at the Enchantment Under the Sea Dance. In fact, there are four of them.
I don't see why not. If the arrival of the original Marty modified the timeline - and we know it did, because of the knockon effects in 1985 - then arrivals of subsequent Martys (Marties?) would have modified the modified timeline. So, yes, first time through there was just one, then there were more.