I would like to clarify your doubt...notice what happens in a reaction between a strong acid and a weak base or a strong base and a weak acid when you take the weak electrolyte in lesser amount... Suppose we cause a reaction between acetic acid and sodium hydroxide...It does feel as though it would be reversible as most ch3cooh would be undissociated... But remember that OH- is continuously coming from NaOH... And the result is that H+ are being consumed very quickly (due to them reacting with OH- and forming water which causes the equilibrium of dissociation of acetic acid to shift towards right (Le Charteliers principle) and so the weak acid starts behaving as a strong acid and does not stop the supply of H+ unlike in a normal solution of acetic acid where the concentration of H+ coming from it is fixed (Ka)...the supply of H+ continues till all acetic acid is exhausted as it is the limiting reagent and not NaOH... So this reaction basically becomes a strong acid and strong base reaction...which is well known to be irreversible... Same is the case with strong acid and weak base where in turn H+ pull the OH- out of the weak base again making it a strong acid strong base reaction... But if you take the weak electrolyte in excess the reaction stays irreversible till the OH- or the H+ coming from the strong electrolyte get exhausted... Afterward the solution becomes a buffer.
But remember this can never be the case with weak acid and weak base as both H+ and OH- are limited... According to me their reactions are always reversible.
Now addressing your doubt... Although CH3COO- is unstable... But it exists isolated in a solution due to NaCH3COO being a strong electrolyte... Remember one thing... Most salts of weak acids are good electrolytes... And it doesn't react with H+ as the CH3COO- so formed against dissociates due to the action of NaOH.