Answer 1

["Maziar","Dokhanian"]

Don't send challenges to random user, send to them who are in top 10, they are always ready to accept.

Answer 2

It all depends on your opponent, whether or not he/she will accept your challenge.

Answer 3

Everyone is free to accept or decline or let expire a challenge.

A already suggested, you can try to challenge higher-lev/ top-of-the leaderboard users. Or users whom you follow and know to be still active on sololearn.