Which is better: congregational prayer at work with five people, or prayer in a nearby Musalla led by someone who makes mistakes in Al-Fatiha?

Congregational prayer does not require a specific place, and it can be performed by two or more people. The larger the congregation, the greater the reward. If it is possible to pray in a larger congregation without disrupting work, that is preferable. If the imam makes a grammatical error in Al-Fatihah that alters its meaning, then it is not permissible to pray behind him. In that case, you should pray with your colleagues, and your prayer with them will be valid and sufficient.