As the signal path can’t utilise the Sparrow DAC for anything other that the headphones connected to it, other endpoints will be reliant on whatever Roon does to support the signal processing through software.
See my follow-up question above - is it sufficient to simply plug it into the mini’s USB (without having to plug my headphones on the Sparrow’s other end) for its high quality DAC-ized stream to be used by any endpoints in the house?
For other Roon end-points, you would need additional DAC’s connected to your Roon core by ethernet or USB. For each of those DAC’s, you need a Roon ready or Roon tested device that would be connected by ethernet to your network, then USB, HDMI, RCA, etc to your sound system or active speakers.
You can do this. Your Roon core (Mac Mini) will send the music back to your router by ethernet, then to your iPad or iPhone by WIFI. Your iPad and iPhone can act as both a Roon control device and a Roon end-point.
Your Roon core can do the first unfold of MQA files. If you want full MQA rendering, you need an MQA DAC. For instance, an AudioQuest Dragonfly plugged into your iPhone or iPad feeding your headphones. If you go this route, you also need an Apple camera adapter between the iPhone and Dragonfly DAC. You can find the Dragonfly and camera adapter on Amazon. Get the adapter that allows you to plug in your charger at the same time.
EDIT: If your Sparrow can do this, forget the Dragonfly.
Roon can work as core decoder. So you have the important first unfold. You do not get the 2nd unfold of MQA, but if you don’t have the frequencies above 48 kHz is not really relevant in this situation.