Salamun Alaikum.
Thank you for your email.
Indeed, as per the Quran, instructions were prescribed in the tablets (l'alwahi) and they were written into them.
The Arabic word 'wa-katabna-hu' (7:145) means 'And We wrote for him'
However, there is nothing to suggest that the instructions were not then taken down into general written form to preserve the tablets. However, the Torah with us today also has additions that were written by other scribes and verses which were written after the death of Prophet Moses. Therefore, not all the Torah today had its source from Prophet Moses.
The Ten Commandments and the laws / instructions (Book of the covenant) are referenced in Exodus. The original tablets, according to the Biblical account were broken (Deuteronomy 9:17) which can also be implied (though not confirmed) by the Quran, verse 7:150. They were then understood to be rewritten on replacement stones and placed in a chest called the Ark of the Covenant. The original tablets of Moses are lost.
There is no proof from the Quran that Prophet Muhammad received guidance on such tablets. It is clear he received the Quran through inspiration and was taken down by noble scribes (80:13-16) in his lifetime.
With regards 7:142, there is nothing in the Quran to prove that this is a reference to Ramadhan and the Hajj fasts.
I hope this helps, God willing.
Joseph.