the idiomatic use of "no more than" and "no less than"
I think COBUILD is misleading you. The constructs are the same for "more" and for "less". "No more/less than ..." does indeed have the connotation of " ... and look how small/big it is". So far, so good. The connotation is much weaker in the case of "not more/less than ..." which focusses more on the literal meaning and the stated measures of age, size or whatever is being described.
About question 2: Take another example [1]
This restaurant is no less expensive than that restaurant.
This sentence is interpreted as "this restaurant is expensive, just as that one is expensive". Why is the expensiveness of this restaurant regarded as the same level of the expensiveness of that restaurant? Suppose that the reason is that the expensiveness of that restaurant is assumed to be the highest in the scale and therefore this restaurant cannot be more expensive. We can generalize and say that the no less X than Y construction implies that Y is the highest in the scale of Xness. Then it seems possible to me that a sentence including the unit no less than like
The guide contains details of no less than 115 hiking routes.
can be seen in the same way because of the similarity to the no less ... than construction, namely 115 is the highest in the scale of hiking route coverage. If we are allowed to see the unit no less than in that way, "the amount is larger than you expected" meaning follows naturally.
[1]Sawada, Osamu. 2004. The cognitive characteristics of the idiomatic comparative constructions: a case of the ‘no more/less...than’ constructions. Proceedings of the 9th conference of Pan-Pacifc Association of Applied Linguistics (CD-ROM), 273-279.
I think this "no" before a comparative is a habit: no longer, no more, no less, no better than etc.
I assume the original formula was "in no way better than". But actually there is no reason that would prevent the negation "not" and occasionally you find "not" instead of "no+comparative". The meaning is the same.
It would be interesting to know where the habit of "no+comparative" comes from. It might even come from Latin.