How can I make use of WP Mobile Detect conditions from within a plugin?

You either have to include/require the WPMD file that includes the function(s) you are using, or just use the globally defined $detect object, like so:

global $detect;
if (! $detect->isMobile() || $detect->isTablet()) {
    // output your share buttons
}

// EDIT
The above conditional is the equivalent of your ! wpmd_is_phone() conditional. However, if you want to restrict this to desktop devices only, you should use the following:

global $detect;
if (! ($detect->isMobile() || $detect->isTablet())) {
    // output your share buttons
}