However, «наш» is not just singular, it’s singular masculine. Feminine form is «на́ша», and neuter form is «на́ше».
When Е or И are unstressed, they are not distinguished in the standard pronounciation: they're both pronounced in the same way
I believe they're both pronounced not in the same way as they're pronounced in stressed syllables.
This is because you're showing -where- they are, not -what- they are. "These" would mean that youre pointg at the thing and telling what it is. Вот would be closer to "this is the location of" I always imagine not pointing at the thing, but pointing at a map, like in "Вот Россия" "here is russia"
Dash is only needed in "X is/are Y" sentences when X and Y are nouns (that is, words naming things, people or phenomena). E.g. Маша — программистка 'Masha is a programmer'. You don't need dash with other parts of speech.
So, you don't need a dash when 'is' joins pronouns (words referring to things without naming them): Я программистка. 'I'm a (female) programmer.' Вот офис. 'Here is our office'. (In English, here is an adverb, but in Russian вот is a pronoun. It's like 'this-thing-here'.) You don't need a dash with adjectvies (words naming qualities) either: Вода́ холо́дная. 'The water is cold.'
На́ше is singular neuter form: на́ше объявле́ние ‘our ad’.
На́ши is plural form: на́ши офисы ‘our offices’, на́ши объявле́ния ‘our ads’.
There’s also a singular masculine form наш (на́ш о́фис ‘our office’) and singular feminine form на́ша (на́ша зада́ча ‘our task’).