Hi, Adrianus. 
You asnswered about existimabant, that impf. ind., rather than futurm fuisse.
You understand that what is conditioned by the nisi nuntii essent allati seems to be existimabant plerique, not futurum fuisset ut oppidum amitteretur.
In Gildersleeve & Lodge,
Conditional Sentences,
3. Unreal Conditional Sentences,
section 597
Remark 2.
it is explained about the case where impf. ind. is used in the apodosis of Unreal Conditional Sentences.
In Unreal Conditions, after a negative Protasis, the Apodosis is sometimes expressed by the Impf. Indic., when the action is represented as interrupted (see section 233)
sample :
Labebar longius, nisi me retenuissem. (see section 254, remark 3)
(I was about to slide down further, if I had not retained myself)
And 254 explains about the Indic. mood.
Remark 1. says,
The Latin language expresses possibility and power, obligation and necessity, and abstract relations generally, as facts ; whereas, our translation often impliesthe failure to realise.
examples of translation : it is possible (but doesn’t occur), I can (but do not), you should have (but did not), it had to be (but it didn’t go so)
samples :
Possum persequi … (I can persequor, but do not)
Longum est persequi … (it will be too long if I persequor, so I do not)
Ad mortem te duci oportebat (you ought to have …, but it didn’ go so)
Volumnia debuit … (Volumnia ought to have …, but she didn’t)
Vivum illinc exire non oportuerat (he should’nt have escaped alive, but he actually escaped)
Remark 3. says,
The Indic. is sometimes used in the leading clause (the Apodosis) of conditional sentences, thereby implying the certainty of the result, had it not been for the interruption…
With the Impf. the action is often really begun :
samples :
Labebar longius, nisi me retinuissem (I was letting myself go on too far, had I not checked myself.
Omnino supervacua eratdoctrina, si natura sufficeret (the Protasis can be positive in the post-Augustan writers) (training were wholly superfluous, did the Nature suffice.)
Preaclare viceramus, nisi Lepidus recepisset … (We had gained a brilliant victory, had not Lepidus received …)
Though it is not written in Gildersleeve & Lodge, the use of Indicative gives an impression of confidently judging something, compared with the weaker confidence expressed by the Subj..
And section 233, in which what you are saying is written,
The Imperfect is used of attempted and interrupted, intended and expected actions (Imperfect of Endeavor). It is a tense of disappointment and (with the negative) of Resistence to Pressure. > [/quot]
examples of translation : was going to …, was about to …, was almost …ed, attempted to … (the result is unsaid), was expected to .. (but was not actually …), would not (resistently)
samples :
Curiam relinquebat > (he was for leaving the …)
Lex abrogabatur > (the law was about to be abolished)
ostendebatur quomodo … > (an attempt was made to show how …)
Aditum non dabat > (he would not grant access)
Though different from the Subj. which expresses the speaker’s imagination as an imagination, this use of the Impf. Indic. resembles it in expressing the speaker’s imagination almost as a fact.
So this Impf. Indic. could be called another Subj..
By the way, I would like to ask you about Unreal Condition in Oratio Obliqua.
Gildersleeve & Lodge gives these patterns.
A.Dico (dixi), te,
… I’ve written too long, (having much time and idle energy since it is Sunday today), I ask you tomorrow about this.