Getting the last X posts, but in ascending order of time

Have you tried the last parameter with ASC like 'order => 'ASC' ? I think it will solve your problem. So your full code will be like-

// get posts
$posts = get_posts(array(
    'post_type'         => 'events',
    'posts_per_page'    => 2,
    'meta_key'          => 'from_datetime',
    'orderby'           => 'meta_value',
    // Here is the main trick happening.
    'order'             => 'ASC'
));

Hope that helps.