1. Can I reuse editorial content for different subdomains?
Yes you can using the subsites module. It provides a page type called 'Subsite Virtual Page' (or something similar) which allows you to share pages between subsites.
2. Can I keep the old site's forum for a while, by including our legacy PHP function
Sure, if you want to do it like you listed. Though you would do something to fit within the template engine but I would think your forum code may not play well with SilverStripe so you might have to install the forum into a subdirectory and tell SilverStripe to 'ignore' just that directory by using .htaccess rules
# ignores the 'forums' folder.
RewriteCond %{REQUEST_URI} !^/forums