Some various questions

1- I use the phrase "out there" exactly as its words literally express. For instance, "there are many different types of that device out there". By "out there" here I mean: the market, the outside part, or in general, in the world!

Is it right, please?

I always use the phrase with that concept in sentences.

2- As yet, I've seen many sentences in which a "singular pronoun" is used in regard with a "plural noun".

I suppose, when the plural noun, at that particular section of the text, can be toyed with like a package, collection, or something like that, we are able to use a singular pronoun for it. Do you agree please? And if so, will you please tender some example sentences regarding this case.

3- What's the meaning of "off" here in this sentence, please?

The paintings were stolen off wall as soon as they were hung.

4- For me, as a non-English native speaker, at times, it's very difficult to figure out the right way of pronouncing the past tense forms of regular verbs (ended in -ed) whether that letter 'e' is pronounced or dropped when speaking, actually. My question is, is there any clear rule on that to teach us how to say those verbs just like you native English speakers? If the rule varies between BrE and AmE, please say its BrE type.