Makes sense, it'll definitely be better if posts-2-posts supported this functionality natively.
Being able to do something of this sort
'name' => 'posts_to_pages',
'from' => 'post',
'to' => 'page',
'connection-order' => 'post-date',
'connection-orderby' => 'DESC'
^ That would really help and this way, we could probably have the connections sorted by basically anything, like custom post meta fields and such.
This way sortable can be set to false so users aren't able to sort the posts and admins such as myself can still define a default order they'd like the connections to be in.
If sortable is set to true, then this still remains the default order, only that now users can sort the order if they'd like. Basically how it is now.
I think this maybe a more long term way to go. I have no clue how it's handled though, so I guess i'll leave it up to you.
But if there is a short term way of mass fixing the connection order to post date as ASC while making get_adjacent_items function working as sortable is set to false would be great.