I generally agree with both Inziladun and Morwen ,but still would like to make some adds.
Question 1.
Concerning the existence or not of Isildur's bloodline,for someone as well-educated as Denethor there must have been somes clues in the records of Ondoher's time about Aranarth,since he was the grand-son og the Gondorian king,afterall.
Question 3.
Concerning the term "disguise",I think it was not a physical one,because unless he could have his entire face hidden,indeed everything on Aragorn was not only crying"Numenorian blood",but also "Nobble Numenorian blood".I believe a shady and rumor-like story concerning his origins is more likely to be the disguise,more similar to the "undercover" term.Perhaps stories as ,for example,half Numenorian from the North(Dail maybe?),being spread around the soldiers with his consent,if not willingly by him,is a scenario.
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