Schengen multiple-entry visa validity question

The two comments are correct:

How can I get a Schengen visa which is valid till June, 2019 so that I need not apply again?

Under your circumstances as a first timer you can't. Apply in Netherlands. Read the visa rules on the embassy website, it answers your questions.

And

You will generally be issued a single entry visa as a first time applicant.

On the other hand, if you had asked about your chances, your question would have been closed as "primarily opinion based." We can't give very reliable predictions about your chances.

When you apply for a visa, you can always ask for a multiple entry visa. As a first-time applicant, you are indeed unlikely to get one. The question is whether you should. Will it increase the chance of a refusal?

I suspect that the answer is no. It might even show your good faith if you have a subsequent short trip planned. As they say, nothing ventured, nothing gained.

Others may disagree, however, and the true probability probably depends on factors you haven't mentioned, such as your relative wealth, the stability of your family and professional ties to your place of residence, and possibly even the identity of the Schengen country you would travel to in 2019.

The conditions for multiple-entry visas are specified in Article 24(2) of the Schengen Visa Code:

(a) the applicant proves the need or justifies the intention to travel frequently and/or regularly, in particular due to his occupational or family status, such as business persons, civil servants engaged in regular official contacts with Member States and EU institutions, representatives of civil society organisations travelling for the purpose of educational training, seminars and conferences, family members of citizens of the Union, family members of third-country nationals legally residing in Member States and seafarers; and

(b) the applicant proves his integrity and reliability, in particular the lawful use of previous uniform visas or visas with limited territorial validity, his economic situation in the country of origin and his genuine intention to leave the territory of the Member States before the expiry of the visa applied for.

In your case, you have no status leading to a regular need for travel to the Schengen area, just a desire to visit the Schengen area a second time, for a completely different purpose, during a trip to a non-Schengen destination. The applicability of paragraph (a) is therefore tenuous. Paragraph (b) may apply in general, but you lack the first explicit element, which is a history of previous Schengen visas.

Can I use my Schengen visa, which I will be getting now, to enter any Schengen country again in June 2019?

If it is still valid, and you haven't used up the allowed entries or duration of stay, yes.

What is the validity of this visa?

It's up to the country issuing the visa. Normally for a first visa it will be just enough to cover your first trip. If you're lucky, they'll grant it for a year, but they certainly won't do that unless you present a good reason for it in your application.

Which embassy should I apply for a Schengen visa: the Netherlands, as my longest stay for current trip is in Netherlands?

Yes.

How can I get a Schengen visa that is valid until June 2019, so that I don't need to apply again?

When you apply, include detailed plans for your 2019 visit and ask for a visa that will cover both visits. Then hope for the best, but don't hold your breath.