The source of saying this סברי formula is the שבלי הלקט in סימן קמ, quoted in בית יוסף אורח חיים קסז ד''ה כתוב. His original formulation is 'סברו מורי'. He explains that when one person wanted to be מוציא others in the ברכה of wine in the middle of the סעודה, he had to get the other people's attention first so they could swallow what they were eating and prepare to listen to the ברכה.
This same idea is found in many places such as תוספות ברכות מג עמוד א ד''ה הואיל, מרדכי ברכות רמז קמה and הגהות מימוניות ברכות ז,ט or ז,י in the Frankel edition, all of which have the גירסא of סברי מורי.
The interesting thing that the שבלי הלקט does is to expand it to קידוש. There is no problem of people having full mouths as you start קידוש (or there shouldn't be) so we could technically say ברשות. The reason we would want to say ברשות is to warn them to pay attention to the ברכה and ask permission to say it on their behalf. However, he says that we avoid differentiating between different ברכות on wine and thus always say סברי. He also says that for bread, we say ברשות and not סברי, contrary to your father in law. My father also says סברי for bread but I think it is a mistaken מנהג, due to thinking that both ברכות with which we begin the שבת meal are the same.
It so happens that when this is quoted by רמ''א in אורח חיים קעד,ח, he says that we say סברי רבותי.
The phrase 'סברי מרנן' is actually in a מדרש תנחומא 's description of how a decision of בית דין is made to kill someone (found in the beginning of פרשת פקודי). It is quoted in the context of ברכות by דעת זקנים ויקרא י,ט as well as טור אורח חיים סימן קעד in the name of his father, רא''ש.
I haven't managed to find the full סברי מרנן ורבנן ורבותי but it is a reasonable assumption to make that people felt they shouldn't just include the Rabbis and other important members of the congregation in with the 'rif-raf' so gave a special term for them all.
The sum total of the discussion is whatever words you use, the intention is to make sure people are paying attention to the ברכה said. How you refer to those people is probably not the end of the world and there seem to be a few different customs in early sources.