BOGEPATROJ is the correct form. BOGEPATROJ means that GEPATROJ of someone became GEPATROJ of another person, by effect of marriage between these people.

GEBOPATROJ would mean a set of BOPATROJ of the most varied people, including men and women (FATHERS-IN-LAW and MOTHERS-IN-LAW). Congress of GEBOPATROJ would include necessarily MEN and WOMEN, having at least one son married or a daughter married.

BOGEPATROJ estas la korekta formo. BOGEPATROJ signifas ke IES GEPATROJ iĝis GEPATROJ de alia persono pro efiko de geedziĝo inter ĉi tiuj homoj.

GEBOPATROJ = aro de BOPATROJ de diversaj personoj, inkluzivante virojn kaj virinojn.

KONKLUDE: GEBOPATROJ estas vorto kiun oni povas formi pere de la esperanta logiko, sed kun tute alia signifo ol BOGEPATROJ.

I think they do stack in sequence, so no, I don't think "gebopatro" is a valid construction.

No - that would mean something like "they live Americanly, in a USA sort of way".

You can turn adjectives into verbs but not, in general, nouns.

Did you mean "Sxiaj bogepatroj logxas Usono." ?

I agree with you. (And since you reported the point, I won´t.) Now, another remark: "bogepatroj" could also be written, if we follow the Esperanto logic, "gebopatroj". Ĉu ne?

Any word that is describing a noun has to match the noun for number and case. "Lia patro", but "Liaj gepatroj" and "lian patron" and "liajn gepatrojn".