I've never seen any purpose in bench bleeding. The usual problem with bleeding Merc brakes is that the fronts and rears share the same reservoir. There's a molded-in baffle at the bottom which allows the front and rear supplies to be segregated. Even with the translucent plastic, it's hard to see if both sides are full. The best way to fill the reservoir is to fill it to the very brim, almost to overflowing, wait for it to settle down, and top off. That way you know the level is above the partition and the two sides are both full. I don't know about toy stuff like pressure bleeders, but if you do it this way, the pedal provides sufficient power to bleed the system, even if the master hasn't been bench bled. Just keep the reservoir topped off until the last, then use the last bleeder to bring the level down to max.

There's no proportioning valve in a 4-wheel disc brake system, proportioning is a natural artifact of the piston diameters. Pressure regulators are only found in drum brake systems. There is a T block at the rear which splits the line left/right, but it has no "smarts".