#1
I presume you means that you want newest orders first?
Yes you can change the order, in admin/send_pos.php, line 778 or so, change the line reads

query=mysql_query("SELECT orders_products_id, orders_id, orders_products_id, products_name, products_id FROM ".TABLE_ORDERS_PRODUCTS." WHERE po_sent='0' AND po_number IS NULL LIMIT $start, $l_odp_nastronie")

to

query=mysql_query("SELECT orders_products_id, orders_id, orders_products_id, products_name, products_id FROM ".TABLE_ORDERS_PRODUCTS." WHERE po_sent='0' AND po_number IS NULL ORDER BY 2 DESC LIMIT $start, $l_odp_nastronie")

#2
Same file, line 33, it uses the 'Email Address (Sent From)' in Admin-Configuration-Email Options.

#3
I presume you want to use the cutomer order number there, not the po number?
It won't accept order_number because the database field is called order_id, not order_number

#4
I assume you mean that you would like to combine multiple products on 1 customer order supplied by the same subcontractor onto 1 email?
The whole package is set up on a per product basis, rather than a per subcontractor basis, but this could probably be done without too much effort.

Regarding whether anyone is going to pick up this mod, it is by no means clear to me that all of it is released under the GPL. No one is going to touch proprietary code, or submit it to the ZC downloads section.
Secondly, even if the code is indeed unencumbered, anyone attempting this will have to ask 'How fluent am I in Polish?'