Is the aforementioned intention for congregational prayer valid, and is it necessary to specify the Imam's name in the intention? Are my previous prayers valid, and is it permissible to adopt the opinion of those who do not obligate making up for prayers missed due to ignorance?

The congregant must intend to follow the imam, by consensus. It is sufficient for them to intend to follow the present imam without specifying him. Therefore, your prayers performed without specifying the imam are valid. As for the prayers you performed without intending to follow the imam, if this was due to ignorance of the legal ruling, then you are not required to make them up, based on the statement of Shaykh al-Islam Ibn Taymiyyah: "His ﷺ tradition regarding someone who did not know the obligation is that they are not required to make it up, because obligation is conditioned upon the possibility of knowing and the ability to act." This is a recognized opinion, and it is permissible to follow it.