Learn English – The use of “Nos.” for multiple objects

The plural form of Mister is Misters, and the abbreviations Mr. and Messrs. respectively (although UK English drops the periods). The odd spelling is because "Messrs." comes from the French "messieurs". So your example would be phrased as:

Dear Messrs. Jones, Smith, Bloggs, and Flintstone

The abbreviation for addressing more than one Ms. is either "Mses."or "Mss."; note that the abbreviation "Mmes." (from the French "mesdames") is used for the pural of "Mrs."

If it's mixed between two genders, use the appropriate honorific for each set and join them with "and". So for instance:

Dear Ms. Smith and Messrs. Jones, Bloggs, and Flintstone

For addressing a married couple with the same surname, a variant may be employed.

Dear Mr. and Mrs. Smith

Ibidem is only used immediately beneath or next to what it is repeating, so there can never be any doubt as to which work it refers to. With that respect I see no problematic ibidem in your examples. If there could possibly be any doubt, do not use ibidem.

If there are two works whose titles are too similar, you must always add the author when referring to either work.

If a book should be titled Ibid. or Ibidem, which may perhaps exist, you would simply add the name of the author. If there is no author, either use [anonymous] or a description of the context in which the work was conceived, like The Priapic Society of Berlin. If two anonymous works titled Ibidem should exist, add the date, place, or editor to distinguish between them. If two works with the same title should appear on the same day with the same editor, use a distinctive phrase from the first page as a subtitle, as in Papal bulls (like In Nomine Domini from 1059) or psalms. If the contents of the two works should be wholly identical, it doesn't matter which one you refer to.

I hope you will be able to sleep tonight with this advice.

[Edit:] Note that ibidem ought to be used only to refer to a work physically next to (and preceding) it. Referring to a work mentioned on the previous page or to a couple of notes back on the same page is unacceptable. One can only add in Ibidems after the lay-out of your book/article has been finalised (if added by hand). I was also taught that "Jones, ibid., 234" is bad practice. Lastly, I often see "Jones (1921), 234", which I think is annoying, because a year is too meaningless and does not serve to identify the work: one should use short titles there ("Jones, Ostracised, 243").