Ok, so you know that Google no longer uses the keyword meta tag. Nor do the others.
The next thing you need to understand (to answer Q1) is that it's not your keyword list. No, it's your (potential) visitor's keyword list. People will find your content using a number of different keywords and keyword combinations, including misspellings, synonyms etc. So get over thinking you can control how people find your content.
What are you're alternatives? Well, that you do have some control over. Ensuring that your page's title contains your (most important) keyword or phrase, that your keyword/phrase is in your headings/emphasis tags on your page, contained within your body copy etc. Equally important is that the links from your own, and other sites, that link to that page, contain the keyword/phrase in the anchor text of the back-links.
Remember, each page should only be relevant to a core keyword/phrase - there's nothing to be gained from trying to bundle them all in to all your pages or your home-page. ANYTHING you do to try and manipulate how search engines view/rank your site/pages, will end up costing you in the long run.
So: Write Naturally on a single topic, per page.
If your page is simply a button and a textfield - why should it deserve to rank? Unless your backlinks to that page can overcome your 'on-page' content issues.