OK, a couple of things I'd do. Pull the plugs and shoot a goodly shot of Break-Free (gun solvent) into each cylinder. Let sit over night. Next morning, I'd put a small squirt of Marvel Mystery Oil (MMO) into each cylinder and put the plugs back in. fire it up and go drive it. You likely have stuck rings ...
What will happen is the gun solvent will soften the carbon "crust" trapping the rings in the lands. The small shot of MMO will put more compression pressure on those rings and force them to move. Once they start moving, they are free. The extra pressure will also force the softened carbon into the crankcase where it will get caught by the filter.
Come back with it fully warmed up and drain the oil. Refill with Mobil Delvac 1300 15w-40 HDEO (or Delo400, etc.) because it's cheaper and the higher viscosity will slow things down. Plus the added detergency will help clean the motor slowly. Cleaning will further free things up.
You could even include a Qt of MMO in the change with the Delvac to also help free the rings while running.
Will this cure it all - no. Will it slow the consumption - yes, very likely
Will it keep you going for a while - yes. It'll buy time to see if dealer will fix the motor
In the end, if not, you at least have lowered your consumption
For filter, I'd do that once a year as suggested. It's not a depleted/dirty oil problem or a metal flaking issue (I hope...). So the filter is a minor concern. Make sure whatever filter you use has a good anti-drain-back valve so the engine sees pumped oil almost immediately (Wix, Baldin, Napa Gold, etc.).
Why put a rebuilt engine in it - because it raises the resale value. If Toyota has to do it anyway, no skin off the owner, and they get more when they sell or trade it in