How does the Schengen visa duration and entry work?

I am an Indian national with an Indian passport and I am planning to travel from New Delhi to the USA and from there to Amsterdam and then back to New Delhi.

I already have a 3 month US Visa and now I'm planning to apply for a Schengen visa to visit the Netherlands. I had a few questions, please help me out.

The current itinerary is,

New Delhi -> San Francisco 20 March

San Francisco -> Amsterdam

Starts on 26 March from San Francisco

Reach London Gatwick on 27 March

20-hour Layover in London

London to Amsterdam on 28 March

Amsterdam -> New Delhi 3 April

I have read that the schengen visa starts from the day you reach schengen area. In my case should I apply for a schengen visa from 27th March or 28th March as I will be on a layover in UK on 27th?

Also I'm hoping to get a Transit without visa concession in UK, but will they need a schengen visa for the same day as well?

Will there be any issues if I book a round trip from New Delhi to Amsterdam just for visa application and then travel according to my plan above later once I have the visa? I was thinking to book through a travel agency for a reservation of 27th March to April 6th from New Delhi to Amsterdam round trip instead, and if I get the visa, then I can plan according to the itinerary above.
Are there any issues if I travel directly from USA to Amsterdam without returning back to India in between?
Does the immigration officer at border check the itinerary we submit to the visa application center? Can the dates of arrival or departure be changed later (provided they are still under the visa duration).
Should I apply for a single entry or multiple entry schengen visa? Does it have any weightage in the visa decision?

Thanks for the help :)

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How long does a Schengen Visa last?

For double or multiple entry (circulation visa) Schengen visas, the validity of the visa ranges from 6 months to 5 years. The holder may travel one or several times for up to 90 days over a 180-day period. The validity of the Schengen visa is indicated on the visa attached to the traveller's passport.

How long can I stay in Europe with a Schengen Visa?

Most visitors (including Americans) are allowed to spend 90 days in the Schengen Area in every 180-day period.

Can I get a 2 year Schengen Visa?

The basic version allows you to spend 90 days at a time within the area over a period of 180 days, up to as many trips in and out as you require. Multiple-entry Schengen Visas are also available lasting one, three, and five years - covering you for any trips within that period.

Does the Schengen Visa reset after 180 days?

So, when you apply for a Schengen Visa, no matter the Schengen Member State, your visa will be valid for 90 days per 180 days. This means that you may only spend 90 days within the Schengen Area, no more than that. If you overstay, you risk deportation or getting yourself banned from the Schengen Zone.

THE SCHENGEN ZONE TRAVEL EXPLAINED - DIGITAL NOMAD TV

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

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