Is it obligatory to eat Sehri to observe wajib or mustahab roza? if I was not able to eat sehri and not able to awake on sehri for observing roza. When I wake up at 8:00am I have not eat anything yet can I perform niyat of wajib roza of my left roza of last ramadhan as well as mustahib roza before going to ziaraat.

Yes no doubt eating Sahri is mustahab not wajib so you can do your fastiong without having sahri. Also if you awake after fajr azan you can do niyat of your wajib left roza or Mustahab roza if you have not performed any act which invalidates your fast.