The guiding principle should be don't use Past Perfect unless you really have to.
The uncertainty occurred after the asking - chronologically, and in the narrative sequence of OP's text. That's what normally happens when you report a series of events...
I did this. Then I did that.
It's grammatically possible, but completely pointless in most contexts, to express that as...
I had done this. Then I did that.
In OP's second example, it's pretty much impossible to avoid Present Perfect with the word must. You could get away with ...but it must be [that] you forgot..., but it's a bit "starchy". Present Perfect is perfectly natural here.
Consider, for example,...
1: You didn't answer the doorbell when I rang, so you must have gone to work early.
2: You didn't answer the doorbell when I rang, so you must have been in the shower.
Clearly in #1, the action in the present perfect clause (you going to work) happened before the simple past (I rang). But in #2, you being in the shower happened at the same time as I rang. That's not a problem in English; if the chronological relationship between actions is obvious from context, it doesn't always have to be made explicit by the verb forms.
For some reason "used to" seems informal to me. I don't think I'd use it (no pun intended) in an academic essay.
But besides that, I don't see a problem with it. Sure, there's ... not exactly an ambiguity, as once you read the complete sentence the meaning should be clear, but what I guess you could call a momentary ambiguity until the reader finishes the sentence. But you could say that about many words. There are lots of words in English that have multiple meanings depending on context. I'm sure we've all had the experience of seeing a word, getting one meaning in our heads, and then reading a little further and having the jarring realization that that was not the intended meaning, so now we have to go back over the sentence and rethink.
So my vote is: In general, don't worry about it and use if freely. But if you're writing a paragraph where you are saying "used to" in the sense of "did in the past" and also "used to" in the sense of "employed for this purpose", you might want to recast the sentence. Like, I think I would avoid writing, "The hammer that Bob used to use to build ..."
Best Answer
Either is or are is acceptable.
Whether to use is or are may depend on:
1 What your teacher says is correct;
2 What style guide you use says is correct;
3 What you think sounds better;
4 Whether you consider "one or more items" to be a singular unit, as in "Seventeen items is all I can afford."
5 Flip a coin.
In this context, it is easy to see one or more items as a singular set:
One or more items in your order is not available.
Some may prefer the rule of proximity, which says to use the nearest noun:
One of more items in your order are not available.
Frankly, the above grates on me, because of the strength of the word one. Or as the Cambridge Guide to English Usage remarks:
The Chicago Manual of Style Online does not have a specific entry that I can find. But in the text of Section 3.21, there is:
If one or more full sentences follow it.., thus: obviously plural.
There is also a question at English Language & Usage:
Which is correct: “one or more is” or “one or more are”?
which has more than one opinion.