From www.focloir.ie I got this sentence:

Is é an rud céanna a tharlódh agus bíodh sé féin i láthair
Translation: The same thing would happen were he himself present

...shouldn't that be 'bhíodh'? Doesn't the past habitual get used here after the conditional tense of tarlaigh, acting as a kind of imperfect tense?

I thought 'bíodh sé' was the imperative.

Can anyone help?! Many thanks to anyone who would like to give advice...

It seems like a typo, doesn't it? But then foclóir goes on to give many examples of conjunctions using "bíodh". Let's see what others have to say about it.

You never know with focloir...this is a site that says (I quote):

"green is a grammatical form of: green bean"

It is "bíodh" and should be "bíodh".
You are right: Bíodh is imperative mood (and never lenited).

But you can describe conditions using imperative mood
(introduced by "agus" when second part of the sentence):

Is é an rud céanna a tharlódh agus bíodh sé féin i láthair

is principally the same as conditional mood with dá:

Is é an rud céanna a tharlódh dá mbeadh sé féin i láthair
(The same thing would happen if he would be present himself.)

Imperative mood in conditions is at least common in Connacht Irish.

Go raibh míle maith 'at as an gcuidiú!