Electrical – Why two breakers (one for the line, one for the neutral) on a single-phase line

Looks fine to me. Consult with the manufacturer to assure the machine is internally built to be able to accept line wire on the "N" terminal. I expect it will be -- they couldn't sell any in North America/Colombia if not.

You have 3-phase "wye" power, Brazilian style. A single-phase load could either go corner-neutral (127V) or go corner-corner (220V).

When you're backfeeding into the grid at a small scale, the same applies to feeds as loads. Since you can attach a single-phase 220V load to A-B, you can also attach a single-phase backfeed there.

It does not matter to which phases your house's loads are attached, because you know of course that a grid-tied solar inverter is totally incapable of creating power for your local use during power outages. That is not what grid-tied inverters do. If you want that ability, things get more complicated.

(NB: All quotes here are from the 2014 NEC, but I know of no significant changes to these passages between 2014 and 2017.)

Methods 1 and 3 are fine...

For a 15A MWBC, both wiring method 1 and wiring method 3 will work and are Code-compliant as described, insofar as the receptacle wiring goes.

but Method 2 isnt...

Method 2, however, violates NEC 300.13(B) as described:

(B) Device Removal. In multi wire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.

as removing the first receptacle of the circuit leaves the second receptacle sans neutral. Fortunately, this mistake won't fry any equipment, but that's only because the second receptacle only gets the second leg's hot.

And all three of these need handle tying to be compliant

In any case, identified handle ties need to be used in order to provide the "common disconnecting means" required by 210.4(B):

(B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.

Informational Note: See 240.15(B) for information on the use of single-pole circuit breakers as the disconnecting means.

and 240.15(B) (specifically, 240.15(B)(1)):

(B) Circuit Breaker as Overcurrent Device. Circuit breakers shall open all ungrounded conductors of the circuit both manually and automatically unless otherwise permitted in 240.15(B)(1), (B)(2), (B)(3), and (B)(4).

(1) Multiwire Branch Circuits. Individual single-pole circuit breakers, with identified handle ties, shall be permitted as the protection for each ungrounded conductor of multi-wire branch circuits that serve only single-phase line-to-neutral loads.