From the manual page 13:

...Therefore one phones output, labelled PH 1/2 on the front, shares the Line (rear) output signal. Although this phones output reaches the same technical specs as PH 3/4, and also has the exact same Extreme Power output stage, it is considered the ‘spare’ Phones output for comparing phones, dual phones usage, and balanced phones operation –...

You might be confused by the explanation, but as you can see, both ‘phones outputs are exactly the same.
They even have to, as together they form the balanced phones output, which would not work if there was a difference.

It’s the rear Line outputs that are different between 1/2 and 3/4:
1/2 has “active electronically balanced” XLR outputs.
The “impedance balanced” 1/4” rear TRS outputs can be routed and used as secondary 1/2 Line out, or switched to be used for the 3/4 channel.

This is why 1/2 is preferred to be used for the main speakers, and 3/4 as main ‘phones output in a typical configuration.

Nothing prevents you from using both ‘phones outputs at the same time.
Even then you can switch to Line out for your speakers without unplugging any ‘phones.

The manual explains the concept and the versatility of ADI-2/4 Pro SE.

It’s heart is one stereo ADC and two independent stereo DAC chips, opposed to other DACs that just have a single DAC.
This explains the ADI-2/4 Pro SE name:
2 inputs, 4 outputs, all fully featured.

The resources of the two DACs are used as optimal as possible.
Both DAC path’s offer similar headphones outputs each, and can have a full fleshed Line output too.

RME’s ADI-2 series devices take full advantage from doing all DSP function AND the volume control in the digital domain, BEFORE the DAC (manual page 99).

This is why you CAN use ‘phones AND Line from one DAC channel at the same time, they just share the same digital volume control.

For convenience there are mechanisms to automatically (by plugging ‘phones) or manually switch/mute front/back, no need to unplug the ‘phones at all in your case, to listen from speakers.

Manual page 39ff shows block diagrams of the various basic configurations, called “Modes”, ADI-2/4 can be set to.
My preferred one for my use-case is Mode “DAC”, shown on page 46.

What you can do is:
• Connect 2 sets of headphones AND two sets of speakers,
• listen to both headphones without switching, or
• switch on the speakers manually or by unplugging ‘phones.

Hope this could give you a little insight into the unique and highly developed  concept of RME ADI-2 series.
It’s different from what you might be used to, and superior to simpler configured DAC / Phones-Amp combos.

An MC phono cartridge with Step Up Transformer behaves like an MM cartridge.
So, yes, if the combo reaches at least 1.25 mV output level, which it usually does.

The ADI-2/4 Pro SE has been shipped from the factory. I am now also waiting for the first customer to report that he/she got a unit.