I see. No, spearheading is a verb in the headline you quoted. The headline means something like Leading the development of an elegant searchable database.You are right in stating it is not an adjective, but it is being used here as such (in my probably wrong opinion)
"Spearheading" is not an adjective.
It's either a verb or gerund in the example.
How so? Can you tell us what you mean please? Explain clearly how you intend to use what you wrote.but it is being used here as such
Under which circumstances is it possible to place the first in a series of adjectives in front of the indefinite article?
No, it isn't. It's a verb being used as a verb.. . . You are right in stating it is not an adjective, but it is being used here as such . . . .