Yes. And the versions of sentences 2 and 3 without"回来” are much more common.

1.﹉ (你)从哪儿买回来这么多东西啊？ 2.﹉都是从那边的商店买回来的。 In sentence 1, 回来 can not be left out because that will make the sentence sound unnatural, or communicate less specifically, where you may possibly mean something absolutely different: (从哪儿买这么多东西啊?): it's impossible to buy so much stuff in the given time because the the distributers around do not have enough stock, one case for example. For sentence 2, 回来 is not essential because it is implied by the context. Words as 来 去 进 出 回来/出去 出来/进去 上来/下去 起来/下去 following verbs or adjectives in Mandarin are traditionally treated by grammarians from China as a sub-class of verbs, called 趋向动词(directional verbs), indicating direction, tendency, result, request(with 起来), etc. For Chinese learners with western language background, it is, though, more convenient to deal with them as suffixs. These elements are attached to verbs or adjectives to give an additional meaning. See also: 百度百科baidu.com: 趋向动词

你终于回来了！ 从哪儿买（回）来这么多东西啊？ 都是从那边的商店买（回來) 的。 The 2 and 3 sentences can be with or without 回/回來。 But these sentences without 回 / 回來 are more concise.

Yes you can, they can omit

They don't seem necessary there.