Which is better: to join the imam in the final tashahhud and complete what I missed, or to wait for another congregation? And is the congregation (jama‘ah) attained if I join the imam before his first taslim?

If you enter the mosque and the Imam is performing the taslim, do not join him in prayer. Instead, wait for another congregational prayer. If you do join him during his taslim, some scholars believe your prayer is invalid, others believe it is valid as an individual prayer, and some believe it is valid as a congregational prayer. According to the relied-upon view of the Hanbalis, one does not catch the congregation if they say the takbir after the Imam's first taslim. However, if one says the takbir before the Imam begins the taslim, they are considered to have caught the congregation. As for the Malikis, they hold that the congregation is only caught by catching a complete rak'ah with the Imam. This is the view preferred by Ibn Taymiyyah and Ibn 'Uthaymin. Therefore, it is advisable to wait for another congregational prayer if possible.