A little correction: Niemcy is the Nominative form plural for both Niemcy as a country, and Niemcy as a people. Th Accusative form plural for the country is also Niemcy, but for the people it is Niemców.This Czech and Polish -y is the ending of the Nominative-Accusative Plural, so literally Niemcy means "Germans". It is absolutely unrelated, both etymologically and grammatically, to the Latin -ia and the English -y (which is the phonetic continuation of this Latin form).
It doesn't. The Russian -ий-/-ij- in these words is a nominal suffix, persisting throughout the declension: Ignatʲij, Ignatʲija, Ignatʲiju… That's the original Slavic approach. The Polish -y is here an adjectival ending, present in the nominative singular but replaced in other cases: Ignacy, Ignacego, Ignacemu… That's a secondary development. The nominative suffix is etymologically the same, but in West Slavic such names began to decline as adjectives.
It is like in English: Italy/Italians, Hungary/Hungarians, Prussia/Prussians.