This is how I would rule/resolve the conumdrum of "how do PH Bards work in 1E".I'm voting 6, simply because, were I to actually be confronted with this puzzle, I would probably unravel it thus:
Bard does indeed create a sort of 'rule of its own'. You are not a 'pre-bard', BUT you can, as either a human or half-elf, dual class, starting as a fighter, and if you both meet the prerequisites to be a bard, and switch to thief within the allotted level range, then you can continue as a thief, without meeting the standard dual classing requirements. You can then switch to Bard within the designated level range window indicated. IF you were to NOT switch to Bard, you would simply stop leveling, unless you met the normal dual class requirements (IE you better be human for starters) since such progress has no rule making it legal.
Once you switch to Bard, there's no more questions to answer, you made it to Bard, you stay a Bard. If, during your advancement, you were to lose the qualifications to become a bard, then again you would still be able to advance in your current class or do whatever, assuming such was legal for other reasons (the dual class rules, etc.). If not, then you would simply stop advancing at all, until you could come back into compliance.
All of the above I would consider to be the 'baseline' from which any additional rulings might be made on a situational basis. That is, the DM is always free to change the rules, and might do so in return for free beer, good quality pizza delivered frequently, or other unspecified considerations.
That being said, I think the revised, cleaned up Bard from Dragon 56's "Singing a New Tune" is a better alternative.
While I'm at it, I also note that the Sage Advice column in this issue, immediately following that article, is entirely devoted to questions about PH Bards. It doesn't really resolve this thread's question, though.