A1. Hardly had they arrived ...
A2. Hardly had the bus arrived ...
B1. Here come they ...
B2. Here comes the bus ...
One can notice a parallelism between A1 and A2, and these sentences are both acceptable English. Exchanging they and the bus does not produce effects under the grammatical correctness aspect.
But, even if one can notice the same parallelism between B1 and B2, the same conclusion cannot be stated because, as far as I know, B1 is not "standard" English.
Can anybody explain why?
I think the "parallelism" here is potentially misleading. Modern English doesn't normally start a sentence with an adverb in this way. We still use A1, A2, B2 in a limited number of constructions, but the grammar involved is no longer "productive".
As regards sentences starting with "Hardly", I would say these are always dated/formal/literary. Thus, for example, "Hardly had I begun" would normally be phrased today as "I had hardly begun".
We still "tolerate" the form here comes X, but it's no accident that OP chose to switch the verb from arrive to come. Modern grammar doesn't allow constructions like "here arrives the bus", except for certain established usages involving certain verbs (there goes the neighborhood is another one). Here's a chart showing how the form Here stand I, for example, has declined over the past couple of centuries, and here's one for Here stands a man showing that it's the same with both nouns and pronouns.
It seems to me here X comes is an even more "fossilised" form than here comes X. But it's been "modernised" by allowing X to be a pronoun, and we've gotten so used to that form we don't like to put the pronoun at the end any more.
Notice that for the vast majority of verbs which can be modified by here, there, etc., you simply can't put the adverb at the front. You have to adopt the modern style and place it after the verb...
"See that building? I worked there". (grew up, lived, studied, etc.).
Whilst in Barrie is technically correct in saying that expanding "Here come they" into something like "here come they for whom we have waited for so long" would make it grammatical, he is also right in saying that it is highly unusual - I'm assuming you're looking for the phrase you'd use in an everyday context.
If this is the case, the main issue with B1 is one of word order; it should read "here they come" rather than "here come they".
This is due to "they" being a pronoun whereas "the bus" is a noun - for another example it would be "here comes the taxi" but "here he comes". Whilst generally pronouns can be directly used in place of nouns (such as "the bus is coming" vs"it is coming" or "Jack's running past us" vs "he's running past us"), this is an exception. Although I can't think of any examples off the top of my head, there are probably other examples of this happening.
In B2 it is clear what is coming. In B1, because the subject is a pronoun, it isn’t. To remedy that, it would be necessary to expand it to something like ‘Here come they for whom we have waited so long’. That would be grammatical. It would also be unusual.
