being allowed to go to shul doesn't prove that its not a regular day of aveilus because an avel is allowed to go to shul on erev rosh hashana for shacharis because the slichos are long
On Purim and Shushan Purim there is no aveilus befarhesya.
No torn clothes. No low chairs. Leather shoes etc.
(If levaya is on Purim or Shushan Purim the avel changes into non torn clothes immediately after levaya, as on chol hamoed)
However, with the exception of going to hear the meggilah, the ovel is to stay at home.
This is all true for Ashkenazim.
Sefardi rabanim tend to pasken likewise because the mechaber, though he is machmir in hil Purim, is mekal in hil avelus (siman 401). Therefore the Kula is Mishna achrona + halacha kdivrei hamekal beavelus.