Salaah

Reciting ربنا و لك الحمد حمدا كثيرا طيبا مباركا فيه in fardh Salaah

Q: Can a person read the following Dua in his Fard Salaah:  حمدا كثيرا طيبا مباركا فيه after reading: ربنا و لك الحمد, in ruku', similarly can he read in jalsah: اللهم اغفرلي و ارحمني و عافني و اجبرني و ارزقني and if he is a muqtadi can he read these two dua's?

Laughing in Salaah

Q: I understand that if one laughs aloud in Salaah, his wudhu breaks, and if he laughs silently, only his Salaah breaks. However, I wish to know what constitutes a loud laugh and what constitutes laughing silently. Also, is a smile considered laughter?

Hearing one's own recitation in Salaah

Q: Question regarding reciting inaudibly. In my case I was praying Dhuhr Fardh and quietly as to hear my own voice, in my last (4th rak'ah) - I was reciting "Ghayril Maghdoubi" at Maghdoobi, I could only clearly hear "Ma--doobi" the Gha wasn't proper so that I could hear, but I moved my tongue to recite it as normal. Obviously I didn't want to recite "Ma--doobi", so I think I inaudibly said the gha, does this matter or affect the validity of the namaz?