What is the ruling on writing the letter Kaf with a hamza on it in verses instead of the small Kaf, and writing (Ya Rabb) with a repeated Raa, and writing (Alhamdulillah) instead of (Alhamdu Lillah)?

It is permissible to draw the final kaf as you mentioned, because the method of drawing letters in the Mushaf (definition) is not divinely ordained. It is permissible to repeat the letter 'ra' in (Ya Rabb) to express urgency in supplication. However, writing (Al-Hamdulillah) instead of (Al-Hamdu Lillah) is an error, and it is better to avoid it and accustom oneself to the correct pronunciation and writing.