I think also, could it be that the plural form is exactly what stresses the politeness?

Consider that kings used to talk about themselves in plural, perhaps calling somebody in plural sort of supposes that they are considered something greater than just a mere man, some sort of acknowledgment of other person's worthiness.

It is almost nonexistant in Latvian in 3d form, except as a joke, but I think I read some literature, which kinda shows that in 19th c or earlier Russian it was also that respectful address used 3d person plural form, such as when servants were talking to somebody external about their master:

- What is his highness doing, Petr?
- They (or Them) are breaking their fast, then they are going to see the races.

Etc.

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Mums skolā mācīja, ka "jūs" uzruna cēlusies no Senās Romas, kad tur vienbrīd vienlaikus valdījuši divi imperatori, tāpēc uzrunāti dsk. un tā tas iegājies, ka augsti stāvošas personas jāuzrunā "jūs". Mīlīga basņa.

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Jā - the Royal 'we'. Pofig par gramatiku - īsts iemesls slēpās tajā formalitātē: addressing the role of formal personages (person - through sound - through the masks the actors wear), addressing the individual and the formal personage together

laikam

varbūt

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