I still like using bards (the angry ones) as siege weapon ammunition against the old pig-nosed, green-skinned orcs.
1e bards would just brush themselves off and go about their day.
(They have ridiculous hp.)
1e bards were a myth. They were written to use the dual classing rules, and the ability score restrictions of those rules along with random generation made 1e bards almost impossible to qualify for.
There were no dual classing rules in 1e, that's a term from second edition. And 1e bards did not use the rules for a character with two classes, in any case. Neither section refers to the other, and the rules aren't the same. That's one of the reasons why the bard has so many hit points, because the bard HD stack with the fighter/thief HD, instead of overlapping.
You're correct that they're a hard class to qualify for, but there is one other class that's harder.
I'll have to drag out my old copy of the 1e AD&D PHB, but I'm pretty damn sure there were dual classing rules for humans (and only humans) and that the appendix with the bard said that they progressed from thief to fighter (or was it fighter to thief?) then to druid/bard (weird wording) exactly as a dual classed character (even if half-elf, but they might have been able to multi-class fighter/thief before dual classing into druid/bard).
Yes, there are rules for humans switching from one class to another. It's not called "dual classing", though (that's a 2eism). In 1e, it's a "character with two classes." You could only switch once, there were very stiff prime requisite requirements, and you had to be human no exceptions. Until the second class's level exceeded the first no new hit dice were gained, and you couldn't use both sets of class abilities freely.
And yes, the bard started as a fighter, then switched to a thief, then switched to the bard. But that's three classes, not two. Also, they're not restricted to humans. Half-elves are allowed. And while a character with two classes has overlapping hp (fighter 7/thief 9 has 7d10+2d6 hp), and that's true for a bard's fighter and thief levels, it is not true for bard levels. A fighter 7/thief 9/bard 3 has 7d10+2d6+2d6 hp (the last 2d6 because bards only have 2 HD at 3rd level). The bard has minimum ability score requirements that aren't high enough to qualify for a character with two classes. The bard always attacks using their fighting level, instead of losing that ability while using other class abilities.
Those are pretty major differences, and there are no cross-references between the two sections. They do have some similarities, which is why a lot of people conflated the two (me included, before I had reason to go through both sections with a fine toothed comb). But they are completely independent sets of rules.
Bards aren't druids. They just study with druids, cast druid spells, and cast as druids of the same level (max 12, or 13 if 23rd level).