The plural of person is people in this situation. Persons seems to refer to physical bodies in official or formal contexts, for example "This vehicle can carry 7 persons maximum." or "No persons admitted without a pass." or "There were drugs found on their persons." or "Missing Persons Bureau"
Unseres could be used for neuter objects (like das Buch) when the noun itself isn't mentioned. Have a look at this example:
Keiner mag dein Buch. Viele Menschen mögen unseres = "No-one likes your book. Many people like ours".
In this way, it works like "our/ours" in English:
Unser Buch ist unseres = "Our book is ours".
You can't write unseres Buch here, for the same reason you can't write "ours book" in English.
Strong inflection, plural, nominative case.
Or, follow this flowchart: http://www.nthuleen.com/teach/grammar/adjektivendungenexpl.html
Or, think about it like this: Viel- is being used as an adjective to describe Leute. Because the adjective is in front of the thing it's describing, it needs to be inflected (the ending must be changed in ways that match the noun). The noun, Leute, is plural. It's also in the nominative case (the subject of the sentence). Plurals in the nominative case use the definite article die (as opposed to der, das, etc.), and so the ending of this article (-e) has to show up somewhere in the inflection. Since there's no article (it's not die ... Leute), then the only place this inflection can go is directly at the end of viele. When the adjective gets the same ending as the definite article would have, it's called 'strong inflection'.
Menschen can also translate to "people" (i.e. it is not only used where you would say "humans" in English). I'm still waiting for a clear explanation though, as I haven't found a good one on the web yet. Someone else already asked this question, though, so you can help prioritise that by upvoting it.
Because viele means "many", not "every". "Many" means "a large number of something" but "every" means "all of something, without exception". Note also that you use "many" with the plural form ("many people") but "every" with the singular form ("every person"). It works similarly in German.
unser doesn't follow the same pattern as dieser? I'm confused on this, what is the distinction here?
Acc: Masc/Fem/Neut/Pl = diesen/diese/dieses/diese
How is it different for unser? Is it a different kind our pronoun from dieser? These sorts of ambiguities are what throw me off! Using that chart above it would seem to have to be "unseres"... of course it isn't; and that's what is confusing me.
Unser in this case is a dependent pronoun whereas dies- would be an article. https://deutsch.lingolia.com/en/grammar/pronouns/possessive-pronouns
Yes, things are not consistent for articles and pronouns or even different types of inflection, but unfortunately these are nuances you just have to learn. Although I don't recommend you worry about this now as duolingo doesn't address this, just remember that relative pronouns (look like definite articles) change in the genitive.
I feel like this is better said with Leute, rather than Menschen. It’s many PEOPLE, not many HUMANS.