I have a flight, MSQ-IST-BJV return in business class for 3 passengers booked directly with Turkish. Due to the unfortunate situation/war in Ukraine the MSQ-IST legs have been cancelled

There is a Turkish airlines codeshare with Belavia on the day of my original flight, that does appear online in manage my booking (but throws an error when I try to select it). I've had a number of calls with Turkish with varying outcomes:
Call 1: Fine to change onto codeshare, said ticket couldn't be confirmed (put onto HOLD), would call back if any problems.. never called back, whatever was done seems to have been cancelled/ignored
Call 2: System was down (Perhaps the agent was just struggling to deal with the booking)
Call 3: Initially said I couldn't get on the codeshare. Then said that the codeshare flights were full in business in the days before/after as well.

So I guess I could try again, but wanted to get a view and any advice on the forum if I'm just stuck...

I could buy the new flight directly on the Turkish airlines website for two, not three, passengers on this route (but for about twice as much money as I originally paid, likely as its very near the date), which seems to imply the codeshare only has 2 seats in business at any point in time. Belavia (B2) will sell me a ticket directly for 3 passengers.

Any thoughts or advice on what to do? The call centre answers very quickly, but I am not sure if I can be explicit enough on what I need, which is I guess:
1. Split the ticket into two. 2. Book 2 adults onto the codeshare. 3. Hope that another seat is released, or else maybe I need to cancel that passenger and book on Belavia, or put the passenger in economy (its a 6 hour flight now rather than 2 hours, due to the circuitus route around Ukraine/Russia)
2. Just accept its a pain and book separate tickets (MSQ-IST-MSQ, IST-BJV-IST) with completely new tickets, but creates some pain of rechecking in bags I imagine
Or? Is there an advanced agent I can try with