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I saw a mother and baby image from https://www.google.co.uk/maps/preview/uv?hl=en&pb=!1s0x487229d947828a81:0x6a25ec0b5e1b4b77!2m5!2m2!1i80!2i80!3m1!2i100!3m1!7e1!4shttps://plus.google.com/106079570079319333340/photos?hl%3Den%26socfid%3Dweb:lu:kp:placepageimage%26socpid%3D1!5slearning+tree+-+Google+Search&sa=X&ei=KPzPUqDXEYWw7AaW3YDgAg&ved=0CNwBEKIqMBU

I would say tags are useful to readers to help them find more of your content on the same topic.

Unlike Chris, I have clicked on them but people use the web and websites in different ways.

If it's useful to readers then do it.

I would add a link from the home page to the rest of your content (the .com site) but make it clear that people will be taken to a new site.

Redirects not required.

The 301 shouldn't be a problem but as stated it might be odd if used on advertising and then people end up on a different domain. It will help you snap up domain name so no-one else can use it though.

If you don't want a duplicate nl site in Dutch, you could keep the nl domain to a single page with Dutch content and then suggest Dutch visitors visit your main .com site. Would be worth having a contact page with address in Holland too.

If you make the nl site a re-gig of your .com site, make the content as different (and appealing to Dutch visitors) as possible.

Although many Dutch can speak English, they can't all, especially older generations so bear in mind when thinking about just serving English content.