Should “Surely I must be wrong” end with a question mark?
The question mark (?) is almost never used in English as a strictly syntactic marker.
You would do better to think of it as imposing interrogative intonation—the characteristic rising tone at the end of the utterance which 'invites' your hearer to respond. It can thus be employed with utterances which have the form of a declaration:
Surely I must be wrong? indicates that you are not entirely confident that you are in fact wrong, and you would like your hearer to provide confirmation.
A: You're wrong.
B: You say I'm wrong? indicates that B finds A's statement hard to believe and demands assurance that A actually said it.
By the same token, the question mark can be omitted with utterances which have the form of a question:
A: Did you turn in your paper?
B: Did I turn in my paper. B echoes A's question in a tone of annoyance, indicating that it's a stupid question to have asked. —For heaven's sake, of course I turned in my paper!
If your sentence is requesting an answer, then end it with a question mark. If it is not, then do not do so.
When we speak English, we naturally employ a rising pitch in our voice over the course of a question. This rising tone is our invitation to the person we're addressing for them to respond in a complementary way; that is, with a declarative statement unmarked by a rising tone.
The question mark is a technological invention that allows a reader to infer the intended pitch of the spoken utterance. Since there is no sound in writing, we use punctuation to mark critical subtleties of oral discourse which would otherwise be lost.
It is perfectly possible for all three of these possibilities to exist:
- Surely I must be wrong.
- Surely I must be wrong!
- Surely I must be wrong?
Only the last represents the dip and rise of the last word’s pitch that signals an actual inquiry inviting a direct answer.
Technically, this is not a question but a statement.
Feel free to rewrite it thus:
I must be wrong, surely.
However, one may choose to add an air of uncertainty by turning it into a question:
Am I wrong?
Surely, I am wrong. Or am I not?
Surely, I must be wrong?
^ All effectively equivalent.
Once it's a question, the "surely" becomes ironic, because you're no longer sure.
Basically, it's up to you.