If Weatherlight Compleated has left the battlefield by the time its triggered ability resolves, you won’t be able to add any counters to it. Whether you draw a card or scry 1 will be determined by the number of counters it had on it before it left the battlefield.

If Weatherlight Compleated is a creature and dies, its last ability will trigger.

If Weatherlight Compleated loses its abilities, the type-changing effect of its second ability will continue to apply to it. This is because effects that change an object’s types are always applied before effects that remove abilities. In that case, if it has four or more phyresis counters on it, it will be a legendary Phyrexian Vehicle artifact creature with no abilities. This is true even if it somehow gained its fourth phyresis counter after losing its abilities.

If more than one creature dies at the same time, Weatherlight Compleated’s triggered ability will trigger once for each of those creatures. As each of those resolve, its controller will take the instructed actions. For example, if it has no phyresis counters on it and seven creatures die, its controller will put a counter on it and scry 1 as each of the first six instances of the ability resolve, then put a counter on it and draw a card as the last instance resolves.