How Important was Lenin's reintroduction to Russia during World War 1 on the February Revolution

His reintroduction? Do you mean his return? If so then yes, it's safe to say that Lenin's return and his April Thesis made all the difference to the outcome of the revolution.

Before this, the Bolshevik Party was quite confused about how to react towards the new provisional government. At first, they published pamphlets endorsing it (no calls for all power to the soviets yet) and in the Petrograd Soviet voted in favor of it's creation, only to pass a motion decrying it counter-revolutionary the next day! Typically, the Bolshevik Central Committee tracked towards the right - blatant endorsement for democratic capitalism in Russia. The April Thesis made all the difference to the Bolsheviks. It set the part on the right course to successfully carry out the insurrection in October. As Trotsky puts it, in his typical poetic fashion;

On April 4 Lenin appeared at the party conference. His speech, developing his “theses,” passed over the work of the conference like the wet sponge of a teacher erasing what had been written on the blackboard by a confused pupil.

It was much more than just the April Thesis, the Bolshevik Party displayed instances of extreme adventurism in June (only just managing to prevent a premature uprising in Petrograd) and July (in which a revolt in Petrograd and Moscow did occur and was crushed and led to the suppression of the Bolshevik Party), as well as extreme caution following the Kornilov Coup (Most of the Central Committee was unwilling to call a revolt without the approval of the Congress of Soviets, Lenin argued that it would be too late by then). Without Lenin there to argue the positions he did, it's unlikely that the Revolution would have been successful; either happening far to early and being isolated in Petrograd, or too late allowing the Provisional Government to regain the support lost during the Kornilov Coup and successfully crushing the revolution.

As for Kerensky... He was the leader of the Social Revolutionaries, a radical social democratic party. Kerensky became the President of the Provisional Government following elections in July. The Social Revolutionaries (most of them anyway, Kerensky included) and the Mensheviks (the other party radical social democrats) were in favour of establishing democratic capitalism in Russia. While Marxists, they maintained that Russia (a backwards and undeveloped economy, dependent on foreign capital and investment) needed to undergo a period of capitalism for it's economy to develop significantly enough - to a similar level of say, Germany - in order for the successful implementation of socialism. Further, the SRs and the Mensheviks were deeply ingrained into the parliamentary process in Russia and just capitulated to the right-wing of the Russian parliament such as agreeing to continue the First World War and prevent land seizures by the peasantry of the old feudal estates. An attempt to destroy the parliament, even to see it replaced by a collection of workers councils - something they ideologically support - was inconceivable. In fact, Kerensky was so opposed to revolution that he instigated the Kornilov Coup by calling upon General Kornilov to seize Petrograd and destroy the Soviets.

If you want to read more on the Revolution I highly recommend Trotsky's History of the Russian Revolution, in particular, chapter fifteen deals with the role Lenin played in setting the part strait after his return to Russia.