"Series": a noun of multitude similar to "lot"

According to Garner’s fourth edition

Though serving as a plural when the need arises, series is ordinarily
a singular noun. But it is also a noun of
multitude, so that phrases such as a series of things take a plural
verb. However, the collocation there {has been – is} a series has predominated
in print sources.

Common nouns of
multitude: lot, majority, mass, minority, multitude,
percentage, proportion, variety.

However, the entry of series of the American Heritage Dictionary reads

When it has the singular sense of "one set," it takes a singular verb,
even when series is followed by of and a plural noun: A series of
lectures is scheduled.

Are these dissimilar usages stylic advice?

Secondly, the inversion after there also chooses singular elsewhere; why is it so?

When the verb precedes the noun percentage, a singular verb is required. That is, a higher percentage of them are, but there is a higher percentage of them.