How to accomodate a balanced only phono stage ?

I think to be truly differential it needs a center-tap 0v.
MC cartridges do not have a center tap so the ground (0v) is floating and does not form part of the signal. The ground path of the tonearm should not be part of the balanced signal - pin 1. The chassis is normally pin 1 with 2 hot and 3 cold OR pin 1 CT (center tap), 2+, 3-.

A mains transformer has a center tap for the 0v which the preamp or amplifier uses as the 0 volt reference and is used as the 0 volt reference for the system.

I don't think anyone produces a Moving coil cartridge which is truly balanced - having a center tap. This would mean having 6 pins.

Just keep in mind that the typical "RCA" cable will have only one conductor for the signal. The "ground" connection in an RCA-terminated cable will typically be carried on the shield that surrounds the "hot" wire. In balanced configuration, there is no "ground". The "hot" wire of an RCA cable fed a balanced signal will carry only the positive half of the signal, and the negative half of the signal will travel on the shield of the cable to the outer ground connection at the RCA plug. Even if you use RCA to XLR adapters, this is the case. Some contend this degrades the signal. Better, IMO, to do a little soldering and change over to XLR, 3-conductor cables all the way.

The main purpose of the pin 1 ground connection on balanced interfaces between powered components is to keep their respective circuit and chassis grounds reasonably common.

Where one or both of the two components has a 2-prong power plug, or is used with a cheater plug, without that pin 1 connection the grounds of the two components may "float" to very different potentials, due mainly to ac leakage paths and stray capacitances in their power transformers. That potential difference may be large enough to saturate the input stage of the receiving component, or at least to exceed its common mode rejection capability.

If both components have 3-prong power cords, then in the absence of the pin 1 xlr connection the grounds of the two components would be kept common only by the ac safety ground wiring, which is not desirable from a noise standpoint.

None of those factors are applicable to a phono cartridge, which has no ac, no power transformer, etc., and which will happily adapt to whatever the ground potential of the phono stage may be.

That said, it may be somewhat beneficial if instead of using an rca cable into an rca-to-xlr ADAPTER at the phono stage input, a twisted pair shielded rca-to-xlr adapter CABLE were used instead. It would be wired such that xlr pin 2 is connected to the rca center pin at the other end; xlr pin 3 is connected to the rca ground sleeve; and the shield is connected to xlr pin 1. Using a cable like that results in the two signal polarities (not signal "halves") on pins 2 and 3 being wired symmetrically (as opposed to an rca cable where the two conductors are not physically symmetrical). That will mean that any noise picked up by the cable will be essentially the same on the two conductors, and therefore rejectable by the receiver in the phono stage.

Regards,
-- Al