Edit--this appears to be a duplicate of this question, which remains unanswered.
Summary: I'm trying to find a way to feed a 50A load panel from two different sources, each with an existing neutral/ground bond. I'm looking for a cheap, safe, code-compliant way to switch the hot legs and neutral on the feeder to my load panels (un-bonding the generator is undesirable). The only off-the-shelf solution I can find is prohibitively expensive, so I'm wondering if my alternative ideas are feasible.
Background: I have a hybrid solar inverter that I intend to use as the main power source for the home, fed by my main service panel. Complicating matters, I have a neutral-bonded diesel generator (Separately Derived System) I'd like to use as a backup power source to the input of the inverter (per drawing), but without switching the neutral at my feeder to the load, it would add a second neutral bond, which is not permitted by code.
Although there's no permitting process to follow in my location, I wish to abide by the NEC.
Since my main service panel has a bonded neutral, it seems the only "right" way to connect the generator would be through a transfer switch that also switches neutral: a 3-pole double throw safety switch seems ideal--aside from the $639 price tag! (I had also encountered and considered a Reliance X Series transfer switch kit, but stock levels are an issue, it's more complicated than what I need, and the price tag is still high, compared to my proposed solution)
I'm looking for another safe, code-compliant (and hopefully, affordable) way to switch neutral on the feeder to my load panels. I'd rather not un-bond the mobile generator if it can be avoided, in order to retain its standalone capabilities.
I considered using a pair of 60A 3-pole safety switches (@$60 each), but would lack a way to mechanically interlock them, so that one is always in the OFF position.
Then I had an idea: the power input to the inverter is switched by unplugging the grid source, and plugging in the generator. Maybe I could do something similar with the feeder of the load panel: wire a 50A receptacle from the inverter output, and a grid-sourced 50A receptacle next to it in case I need to bypass the inverters (again, per drawing). Then, a matching plug would feed the load panel. *This would be a cheap, safe, and durable solution, but I'd like to know if it could be implemented in a code-compliant way.
If that's not possible, I've considered using a 3P contactor ATS in a DIN rail box, which would still be <$100, but I think I'd prefer the simplicity, durability, and UL listing of a plug and pair of 50A receptacles. An RV transfer switch appears to be a similar alternative solution, and has ETL certification. But can it be used in residential house wiring within the constraints of NEC?
edit/update -- Although I no longer intend to try to switch neutral for a Separately Derived System generator, I'd still really like to know if the 50A plug/socket or RV transfer switch would be disallowed by NEC.