Should the worshipper (ma'mum) wait for the rest of the worshippers to say the takbir before the opening takbir (takbirat al-ihram), or should he say the takbir immediately after the imam's takbir? And should the worshipper move to pray beside the imam if the prayer of the other worshippers becomes invalid?

It is not permissible to interrupt an obligatory prayer after the takbir on the pretext of praying alone in the row, because the prayer of an individual behind the row is not invalidated according to the majority of scholars. As for the Hanbalis—who consider it invalid—it is only invalidated if one prays a full rak'ah alone.

If the prayer of the congregants is invalidated and you remain alone, try to join the row or stand to the imam's right. Otherwise, continue your prayer, as it is valid according to the majority. The Hanbalis would require you to intend to separate from the congregation due to the excuse. The fatwa (definition) is to continue praying with the congregation even if you are alone.