Prayer

No need to say Ameen for both cases mentioned.

You are giving greetings to the recording angels from the right and left side and also to the congregation if you are praying in one.

Assalamu aalikom There are prescribed Sunnah prayers and there are general voluntary prayers. You can pray as many rak’ah as you want of the voluntary prayers.

Wa alaikum salaam, If you performed a new wudhu for maghrib and Isha, your prayers are valid.

Wa alaikum assalam, If this was in the final tashahud, you prayer is invalid. If is was in the first one, you should offer sujood Assahu.

Wa alaikum assalam, It is mandatory to cover the shoulders but if one doesn’t, he is sinful but the prayer is valid.