The formal stance of the dev team is that we write to what's in the OEL books as published by CGL or in errata that is marked as Official, either in a forum thread or in the errata PDFs. What he intended to write is - as far as our development process is concerned - irrelevant until it gets flagged as errata.
As for RAW, we are provided with two pieces of cyberware; the Modular Connector, which allows for the quick mounting and dismounting of modular limbs. and the Modular Limb, which as described is a cyberlimb that can be connected to a Modular Connector. Not that it has an integrated modular connector as Wakshaani apparently intended, that it can be connected to one. No cost or availability change is provided for making a standard cyberlimb modular, so there is no content that indicates to us that that can be done.
I will concede that the implementation of connectors is a little bit rocky in that the connector itself has the whole essence loss of the connector and the limb and the modular limb itself has zero Essence but that's largely a matter of practicality on our part. I didn't feel comfortable with the idea that removing an arm costs 0 essence, installing a modular connector in the same point costs .3, but then connecting a limb to it costs an additional Essence and then leaves an essence hole when removed.
Finally, your customdata change will not work the way you want it to. You won't be able to detach the limb, attach a new one and have it change your stats, because you're missing a <mountsto> node on each that tells Chummer that the limb starts detached and that it's allowed to have the limb connect to a mount. Essence will decrease for each additional modular limb you purchase, because as far as Chummer's concerned you're installing them into your body. HOWEVER, what you will find is that if you were to install a cyberlimb onto a modular connector using the mountsto node, your Essence would go back up, even in career mode.
tl;dr There are no rules that state that regular cyberlimbs are able to be mounted to modular connectors and we implemented the rules provided to us in the least insane way possible.