Tawatur Not Recorded
Also- the first generation believes that the prayer doesn’t have to be prayed in a specific way. They just prayed this way to respect the prophet SAWS. Now as time went on- people just assumed that this was the fard way to pray.
That would also come into question then surely?
The ijma and tawatur starts from today’s time by seeing current ijma of Muslims ilm and how common muslims are practicing it and from where they derived it from (their tawatur). It is itself an independent source of knowledge and most of the times, is free of any doubt. But if someone wants to trace ijma of Muslims back, then fiqh books can be consulted which would be documenting ijma of Muslims ilm of their time. On the same basis, let’s say that we have established the same ijma and tawatur to the nearest of the times possible (e.g. 100 years till the time of Prophet(sws)), and it all checks out, then there is no reason to believe based on estimation, probability and all the available evidence, that someone in those 100 years would’ve initiated something of his own that no one else could do for the rest of 1300 years moving forward (the chances are negligible).