Yes, it is a verb. Here, it means "get" or "achieve." The "instead" is used to relate this sentence to the previous sentence. I'm not sure what the previous sentence was, but let's say it was something like "I avoided two bad options." I avoided two bad options. I secured instead a UK-wide temporary customs arrangement – avoiding both a hard border on the island of Ireland and a customs border down the Irish Sea. =I avoided two bad options. INSTEAD, I secured a UK-wide temporary customs arrangement – avoiding both a hard border on the island of Ireland and a customs border down the Irish Sea. =我避免了两个糟糕的选择。我反而实现全UK临时海关安排, 从而又避免将北爱尔兰与爱尔兰共和国分开的海关边境, 又避免将全爱尔兰于大不列颠分开的海关边境。 Hopefully my Chinese sentence made sense. It was a very difficult sentence to translate. I'd welcome any corrections to my translation.

Yes. It's a verb. When "secured" and "instead" are used together like this, it means that the writer considered another option but chose this option.

A verb. Here, "to secure" means to receive, to obtain, to get.

Yes, "secured" is a verb in this sentence, with the following meaning: Meaning: Succeed in obtaining (something), especially with difficulty. Example: "the division secured a major contract"