Why no 'a' in front of ollaimh
Because ollaimh begins with a vowel
Why leibh instead of leat?
Leat would be used with a friend or some other less-formal scenario. Leibh is for someone in a senior position, an older person or similar more formal occasions.
What's the difference between 'mar sin leat/leibh' and 'tioraidh'?
Tìoraidh is much more colloquial and casual. Mar sin leat/leibh is much more formal.