1) The network protocols are not compatible, therefore you will not be able to use any of the products in the "other" world.
2) PreSonus has no product like the P16-I analog-digital converter, because a 16M is not limited to 16 input channels. You can patch any of the 64 network input channels to the 16 channels of an EarMix 16M, but you can also patch flex mixes (auxes, matrixes, subgroups) to the 16 channels or you can use a mixture of both input sources.
3) If you want to send not more than 16 audio inputs to an EarMix 16M, the rack mixer SL 16R can be used as A/D converter. Simply connect both units with a CAT5e/CAT6 cable and use the built-in AVB splitter to connect more EarMix 16Ms.