Both Instrumental forms of the word дочь are correct. You wouldn't get a bad mark, if you used the less common one
дочерями. The situation with the genitive plural forms of год and человек you've mentioned is however more complicated: they aren't interchangeable, but when I first saw your question I failed to come up with any rule that would comprehensively explain why in such and such circumstances a particular form is preferred over the other. Fortunately, I've found an explanation on Wiktionary that seems to cover every possible usage of a genitive plural for those two words:
https://en.wiktionary.org/wiki/год#Usage_notesUse the appropriate case of год after simple or compound numbers ending in оди́н (odín) (1), два (dva) (2), три (tri) (3), or четы́ре (četýre) (4), except for 11, 12, 13 and 14 (and 111 - 114, 211 - 214 etc.) - for these and all other numbers, and with adverbs of quantity (мно́го (mnógo), ско́лько (skólʹko), etc.), use the appropriate case of ле́то (léto).
After numbers that must be followed by a nominative or genitive singular, the appropriate form of год is used. After numbers that must be followed by a genitive plural, the form лет is used. When the number is in the dative, instrumental, or prepositional case, the appropriate form of год must be used. Examples: оди́н год (nominative singular), два го́да (genitive singular), три го́да (genitive singular), пять лет (genitive plural); but бо́льше трёх лет (genitive plural), бо́льше пяти́ лет (genitive plural); and пяти́ года́м (dative plural), пятью́ года́ми (instrumental plural), o пяти́ года́х (prepositional plural).
https://en.wiktionary.org/wiki/человек#Usage_notesUsage of человек with numerals: The genitive plural form человек is used only after whole numbers (tens, hundreds, thousands, millions, etc.), numbers ending in -ь and after не́сколько (néskolʹko), ско́лько (skólʹko) (also - ско́лько люде́й), and sometimes after мно́го (mnógo). Use genitive singular of человек after simple or compound numbers ending in два (dva) (2), три (tri) (3), or четы́ре (četýre) (4). Use nominative singular of человек after simple or compound numbers ending in оди́н (odín) (1). Otherwise plural of человек is a form лю́ди.
The derivations, e.g. получелове́к (polučelovék), and compound words with человек, e.g. сне́жный челове́к (snéžnyj čelovék) or челове́к разу́мный (čelovék razúmnyj), do not necessarily follow the same usage.