I just want to understand: if you refer to a single person formally, do you still have to use the past tense form as referring to one person? So translating "Mister, could you ...?" would be "Pane, mohl byste ...?" and never "Pane, mohli byste ...?"? In Russian we always use the plural form in such situations so I found that a bit unusual.
It's "byste si" vs "by sis." The difference is 2nd person pl/2nd person sg formal vs 2nd person sg informal.
Note the "regular form" of 2nd person sg informal is "bys." However, if there's a reflexive pronoun used, the final "-s" moves to the pronoun: "by sis." Just like in this case.