subdomain or simpler solution

is there an easier way to get a theme for a single page?

Yes. Page Templates.

http://codex.ww.wp.xz.cn/Page_Templates

You said you wanted something EASIER than multisite which I took to mean instead of Multisite. A page template is (relatively) easier. And yes you can use it on Multisite.

If you can’t make a child theme, though, don’t use Multisite. You’re not ready.

You have options.

1) Undo Multisite: http://wpmututorials.com/how-to/how-to-disable-multisite/

2) Keep Multisite and use a custom front page (which by the way, has nothing to do with Multisite at all, and is standard WP stuff)