Dangling modifier or gerund

"Dangling modifier" is only applied to participial phrases without overt subjects. These are normally adverbial, and they may be present participles. Though not gerunds. Gerunds are noun clauses, headed by -ing verbs, and their subjects are either present, or determined by the syntax:

• Bill's arriving two hours late turned out to be bad luck for us.
• Bill was cautioned about arriving more promptly next time.

So never mind danglers; they're not involved. Instead, the concept of "passive" seems confused here. E.g,

As far as I know, the participle doing is acting like a Gerund here, and therefore,
I think, its passivisation is also possible:

• Most of your time is wasted doing unnecessary things. Am I right?

No.
First, the doing clause is not a gerund, but an adverbial participle, meaning while doing unnecessary things.
Second, even if it were a gerund, that would say nothing about its passivization.
Third, the passive is wasted in the example sentence is in the main clause, not the doing clause.
Fourth, the doing clause is active, not passive, in all the given example sentences.

In the other examples given, it's the same phenomenon -- the -ing clause has nothing to do with the passive, because it's in a subordinate clause, while the passive is in the main clause.

Active: I spent a lot of time cleaning that room.
Passive: A lot of time was spent cleaning that room.
Active: You can adjust the temperature by turning this button.
Passive: The temperature can be adjusted by turning this button.

Note that by turning this button and cleaning that room do not change to passive; only the main verbs.

Finally,

• Stir the ingredients until well blended.

In the last example, I think a being is missing. For example,

*Stir the ingredients until being well blended.
To make the second part a Gerund phrase. Am I right?

No again. All the other examples were passivized in the main clause. But the main clause here is an imperative. Imperatives have no passive. So you're trying to passivize what's left, but well blended is not even a clause, certainly not a participle or gerund. No verb, right? No verb, no passive. So that's right out.

What's happened is that until well blended comes from until (they are) well blended by Whiz-Deletion. So your intuition that there should be some form of be is right; but not the -ing form.