If we place a bet with only one selection, we should not send anything in these 3 parameters, right: lastForeignEventStartTime, lastForeignEventEndTime…

stake_multiplier?

Correct, you don't send these 3 parameters for single (1-selection) bets. Especially stake_multiplier, you generally shouldn't send at all unless you have a very specific case where the multiplier shouldn't be the default one (= 10000).