Prayer

This is permissible.  

Yes, this is wrong and he must be advised and informed that our Prophet, sal Allahu alaihi wa sallam, did not do this. Never the less, his prayer is valid and correct but he has left the sunnah of reciting it silently.

If you are referring to the second Jum’ah prayer being performed in the same masjid, if you are going to join the imam in the second rak’ah, this is mandatory for you to do so.

You should join.

What is Description of the Prophet’s prayer ?. Praise be to Allah1 – Facing the direction of the Ka’bah 1-                 When you stand up to pray, face the direction of the Ka’bah wherever you are, in both fard (obligatory) and naafil (supererogatory) prayers. This is one of the pillars of prayer, without which prayer is […]

This is not true. It is a major sin and if you skip three Jum’ah, Allah will seal your heart but you will remain a Muslim.

Yes, you can.

Yes.

If he is travelling, he should shorten his PRAYER.