Create CloudPages to collect and subscribe users is the correct answer. CloudPages is a feature that allows you to create web pages and landing pages using Marketing Cloud tools and data. You can use CloudPages to collect information from users who visit your pages from social channels, such as Facebook or Twitter, and subscribe them to your email lists or data extensions. You can also use CloudPages to create preference centers, profile centers, or unsubscribe pages for your email subscribers. You can learn more about this in the
[CloudPages Basics] module on Trailhead.

To identify the subscribers who have opened an email in the past year, you should use the _Open data view.
This is a system data view that stores information about email open events for all subscribers in your account. You can query this data view using SQL to filter by date and email name or ID

A double opt-in at signup is the best way to make sure email addresses are valid before adding them to the audience, as it requires the subscriber to confirm their email address by clicking on a link sent to their inbox.
This ensures that the email address is not fake, misspelled, or belongs to someone else. A double opt-in also helps to build trust and consent with the subscriber. A Smart Capture block or a CAPTCHA validation would not verify the validity of the email address, but only capture or filter the input from the form.

increasing the font size (over 16pt) to make the content easier to read is a best practice for improving the accessibility of email design. Larger fonts can help users with low vision or cognitive disabilities to access the email content more easily. Reducing line spacing to fit more content on the screen and reduce scrolling is not a good practice for accessibility, as it can make the text harder to read and follow. Removing role="presentation" from layout tables is also not a good practice for accessibility, as it can cause assistive technologies to ignore the table structure and read the content in an incorrect order.

One of the best practices to increase email subscribers is to collect emails at offline activities, including in stores1. This way, NTO can capture the interest of potential customers who are already engaged with their brand and products. NTO can also offer incentives, such as discounts or freebies, to encourage customers to sign up for their email list. By collecting emails at offline activities, NTO can build a permission-based and relevant email list that can improve their deliverability and engagement rates.
Sending an SMS to NTO's entire customer database with a link to subscribe is not a good practice, as it may violate the consent and privacy laws of different countries2. Moreover, sending an SMS to customers who have not opted in for SMS communication may annoy them and damage NTO's reputation. NTO should only send SMS messages to customers who have explicitly agreed to receive them and provide them with an easy way to opt out.
Sending an email to NTO's entire customer database asking for referrals is also not a good practice, as it may result in spam complaints and unsubscribes3. Asking for referrals should be done in a targeted and personalized way, based on the customer's behavior and preferences. NTO should also provide incentives and clear instructions for customers who want to refer their friends and family to NTO's email list. References := 1: Optimizing Marketing Cloud: Best Practices for Managing Inactive Accounts - Salesforce 2: How You Can Keep Email Subscribers Engaged | Salesforce 3: Email Marketing Best Practices - Salesforce.com