Since there currently isn’t a maximum stay and passenger types don’t have a preference for a particular stay duration yet, a single daily flight/connection should suffice for a return booking. That’s in principle, of course. Concrete circumstances might make it more difficult (like on very competitive markets with loads of connections).
Depends. Generally speaking, a return connection requires the so-called plating carrier of each segment to be the same. At the moment, this is determined based on the longest leg of a segment. So as long as your mainline carrier is the plating carrier in both directions, there shouldn’t be an issue.
What does happen though is that a connection of different carriers doesn’t qualify as “direct” anymore for the purpose of Neutral Display Order, which matters internally for connection culling…if there are too many viable options, connections are culled before they are rated based on said order, meaning non-stops and directs get priority over connections between different carriers.
No, not at this time, as they would go against what this change is trying to achieve. Not ruling out that one-way requests might be added in the future, but even if that was the case, I’d assume those requests would be relatively rare. Nothing to build a business on, anyway 
I wouldn’t go that far just yet. As said, this is a first stab and things might (and likely will) still change.
This is a fair point and I see a similar risk. That said, I also see it as a piece of the puzzle to reach the long-term goal of better differentiating business models in AirlineSim. As it stands, interlining and connections in general are far too easy (imo). Nobody would even consider an LCC model without actual connections because it only has downsides. Transfers are essentially free and just happen automatically…one doesn’t even have to put in too much effort to schedule clean waves and planes still fill up with random internal and external transfers. As convenient as that is (or: because it is) I’d like to change that in the future, by making connections generally more costly and hard to achieve, but also by adding new features (this particular case makes codeshare sound more attractive than ever 
)
With over 130 aircraft, you airline group doesn’t appear to be on the small side either . But yeah, as said, as things stand at the moment, especially in an existing game world, this will be easier for large airlines that do not rely on interlining and happen to have the right wave structure. Not sure though how it would play out in a fresh game world, where everyone would have to build up a network under the new rules.
Firstly, Paine is a cheaper, experimental preview game world for a reason: Changes (including drastic ones) can and will be introduced at any time. Secondly, and as said above, I wouldn’t jump to conclusions too fast. If the flight is running at 0% tomorrow when it would typically run at 100% and the change was only introduced yesterday, chances are that flight was already getting lucky before
But again: This new system runs since yesterday. There will be bugs, there will be things that needs tweaks and optimisations. For example, I am pretty sure already that the return connections need to be sorted into time buckets as well because otherwise the connection culling might be too strict for inbounds.