You are not completely off the mark, but there are some misunderstandings.
2018年、SNSで実施した1294人の読書のアンケート
As you guessed, in this quote "実施した" is most likely a transitive verb, with no subject corresponding to it, so someone (not expressly specified in this quote) conducted it, and if it was "実施された", it would mean "was conducted". However, verbs ending in "する" are not always transitive, as we will see below.
iPhoneが再起動した場合Touch IDを利用するには...
Here, "再起動した" is used intransitively, with the が-marked "iPhone" being the subject. If it was "Iphoneを再起動した場合", then "再起動した" would be transitive (which it can be without a problem), with no subject corresponding to it present. You can also say "iPhoneが再起動された場合", in which case "再起動された" is a transitive verb in the passive form.
Here is the takeaway point: There are する-ending verbs that are only used as a transitive verb, and there are する-ending verbs (called "labile verbs") that can be used either as transitive or as intransitive (or more precisely, "unaccusative") verbs.
"実施する" is nearly exclusively used as a transitive verb. ("アンケートを実施する" and "アンケートが実施される" are fine, but many people may find "アンケートが実施する" strange or ungrammatical.)
"再起動する" can be used as a transitive or intransitive verb. ("Iphoneを再起動する" and "Iphoneが再起動する" are both good.)
彼らはウォッカを混ぜたオレンジジュースのグラスを彼に与えた。
Right. The "混ぜた" here is a transitive verb, though the person/people who did the act can be (one of) 彼ら or some other party. (The topic-subject "彼ら" corresponds to the verb 与えた.)
Here's another takeaway: In Japanese, there exist pairs of related verbs whose distinction is due to the difference in transitivity. The 混ぜる(transitive)-混ざる(intransitive) pair is one of them. Other examples include 始める-始まる, 決める-決まる and 見つける-見つかる. But do note that the use of intransitive verb does not entail the lack of a willful actor in the event it describes. "ウォッカが混ざったオレンジジュース" can very well refer to a glass of orange juice someone intentionally mixed some vodka in, and so can "なくしたスマホが見つかった" describe a situation where someone searched high and low for the phone.
選択された特定のWi-Fiネットワーク
Yes, if it was "選択した特定のWi-Fiネットワーク", it would mean "designated Wi-Fi network [unmentioned entity (it can be "you", as the case may be)] chose, (or closer to "have chosen" if you read the た-form as completive rather than preterite).