1. Yes, kuperbaiki is short form of aku perbaiki. 2. It's same

Hello Ku- indeed is short form of aku But here it is. The short form is usually used only when it is placed in the middle or in the end of the sentence. When place in the front, aku is usually used. Ex : I fixed the car yesterday. Aku perbaiki mobil kemarin (natural) Kuperbaiki mobil kemarin (unnatural, except in song or poem to make sentences more beautiful or maybe for rythm necessity). Mobil sudah kuperbaiki Mobil sudah aku perbaiki ( both sound natural) That's it. Alasan kuat and Alasan yang kuat both mean the same in meaning and nuance in your case, and in 90% cases you will encounter. Except, in case when you use it to mean a bit a comparation Example when you are in a store and about to pick a shirt to purchase Aku suka baju wangi would have a bit different nuance than Aku suka baju yang wangi. "Aku suka baju wangi" will normally means "I like shirt that smells good" While, "Aku suka baju yang wangi" would have the nuance "I like shirt that smells good (compared to the other shirts which might not smell good) That's it.