Not really, no. The point of the separate feeding phase is not that it is the only phase that feeds, it is that it is the only phase that feeds on that foodstuff. That increases the total amount of resources available to the species greatly.
Caterpillars for example have chewing mouthparts adapted to break up tough solid matter. Moths OTOH have a drinking tube for a mouthpart. They are quite useless at exploiting anything other than solid food. If the organisms decided to utilise a single body type they would need to make a choice between living exclusively on leaves or exclusively on nectar, they wouldn’t be able to utilise both.
That is a very undesirable state of affairs when you realise that most insects have huge juvenile mortality. There are effectively 100 or more juveniles for every adult. If all the juveniles and adults ate the same food there simply wouldn’t be the resources to support the adults any more, and reproduction would suffer.
Therefore those organisms that evolved in such a way that their offspring didn’t compete with other members of their own species until they were ready to reproduce had a massive advantage.
That’s one of the tough questions, and the answer is a long way form certain. What appears to have happened is that the ancestral insects originally had a form on hatching that was similar to but not identical to the adult. This is still seen in a lot of insects today and the juveniles are referred to as nymphs. In these first insects there was a more or less constant change between the nymph through to the adult form, with each moult producing a few more changes.
Later insect types became decreasingly divergent form this pattern, and all living insects that don’t outright metamorphose go through at least 3 fairly distinct changes. The fist is the pronymph, which is basically designed as a specialised crawling insect. That seems like a sensible enough mechanism evolutionarily, since only the final two moults can produce wings in insects there is little point maintaining a flying form when there is no chance of flight. Far more sensible to have a form that is well adapted to crawling. Following the pronymph modern insects produce a nymph. The nymph is the essentially a juvenile form of the adult, complete with wing buds etc. It is essentially a form that allows a transition from crawling and hiding to flying moving. Then of course finally there is the winged adult form.
The transition from pronymph to nymph to adult is under the control of a complex series of hormones. What appears to have happened is that within some lines of insects the hormonal changes that produced the transition between the three ancestral body types became gradually more dramatic. Instead of the pronymph being replaced over a series of several moults by the nymph which in turn was replaced by the adult the pronymph only moulted into larger pronymphs, and then in one very dramatic moult the pronymph produced a nymph form which then existed largely unchanged and then in trun produced the adult in one spectacular moulting event.Today that has been taken to the extreme where the pronymph is the caterpillar form, and the nymph form has been entirely reduced to a pupa.
It has been proposed that the original impetus for that transition was to free up m ore food for the pronymph forms. By allowing them to exists in, say, a flattened form or a burrowing form they could exploit food in crevasses or underground that would be lost far more rapidly if they began an immediate transition towards the adult form. The ideal tactic therefore was to extend the pronymph form for as long as possible during the spring and summer, and then only enter the pronymph forms during a period of aestivation during the winter, fuelled entirely by internal food reserves. Then when the warm weather returned the pronynmph emerged and went through one or two final moults to produce the adult. In that manner there was limited competition between adults and juveniles even without complete metamorphosis.
By a series of fairly gradual changes, each improving on the efficiency of each stage the current system of total metamorphosis was produced.
