During Purascharana, when the mantra is chanted at high speeds, there is a chance that Mantras are repeated incoherently. Are these chants of any use? Doesn't the Mantra's Deity know that the chant is directed towards Him or Her and accepted anyway? Or is Mantra Sabda Spashtatha (Clarity of Sound of Mantra) of prime importance with Guru Mantra.

Swamiji says:
There are many aspects of our worship to which we are required to pay attention simultaneously: asana, pranayam, intonation, meaning, method, order, etc.
If we make mistakes, which invariably we will, then we beg forgiveness. BUT we ware trying with sincerity. That is the greatest exemplification of our devotion.
How many times did Shankaracharya write: I don't know mantras, yantras, tantras, worship, etc. But I am going to keep on trying to worship you anyway.