No. «А» implies some contrast, while there's no contrast in this sentence, you're doing the same thing.
"Я усміхаюся, а ти ні" (I smile, and you don't) - here "а" would be appropriate.
усмішка and посмішка - yes, the same...
but улыбка and усмешка - very different... улыбка - it is positive... усмешка - negative meaning
усмешка - is incorrect... ухмылка - is more exactly word
Is there significance to the -ся ending on both words? Is this irregular, typical of a certain class of verbs, something else?
-ся is a reflexive postfix. It can be used:
- to express action directed at oneself (ми́ти 'to wash <someone>' — ми́тися 'to wash <oneself>', голи́ти 'to shave <someone>' — голи́тися 'to shave <oneself>'),
- to express a passive action (чита́ти 'to read' — чита́тися 'to be read'; this usually only works for imperfective verbs),
- to express 'each other' (свари́ти 'to scold someone' — свари́тися 'to have a quarrel, to scold each other'),
- to express an action direction at an unknown object or a habitual action (куса́ти 'to bite <someone>' — куса́тися 'to bite <doesn't matter whom>', e.g. соба́ка куса́ється 'the dog bites').
Note that it’s attached after the verb ending. (In some dialects, ся can be placed before the verb, and then it’s written as a separate word, but it’s not used this way in standard Ukrainian.)
As for усміха́тися 'to smile', it’s simply not used without -ся.
(In slang, усміхнути without -ся is sometimes used to mean 'to make <someone> smile', as in «мене усміхнуло» '[it] made me smile', but this usage is very colloquial.)
Please explain why the two cases of SMILE have the different spelling but in the audio the Ю and Є are making the same sound. I am sure that I learned that they both make different sounds and that Ukrainian is a phonetic language without deviations or exceptions unlike English.