With an RS422 adapter, Tx+ to nmeain+ and tx- to nmeain- would do the trick in a proper and safe way. I would not worry about the seemingly contradictory messages. A responsible electronics engineer who is afraid to lose his job might give you this advice as well.
However, it can probably be done much simpler. You found that service manual (well done, I couldn't), and it gives the following schematic for the NMEA input circuit (see below). This means that your input is indeed properly isolated, like it should be, and you could get away with hooking the +3V3 of your raspberry to nmeain+, and your txd to nmeain-. I'm willing to risk my own st2000+ to check this advice before you attempt it yourself, but you have to wait a bit because it's at the boat at the moment.
Big disclaimer here: this might only work in this particular case and is definitely not reversible: if you try to hook a nmea-out to your pi's rxd this way you are very likely to blow your pi. If you don't understand the electronics that sit behind an input like this, you'd better stick to using safe, pluggable components, like the USB to NMEA0183 module. This might require you to tone down your ambition levels a bit, but they are likely come down anyway after so many blown electronics ;-). But I like your tenacity and don't want to leave you hanging.
Cheers,
However, it can probably be done much simpler. You found that service manual (well done, I couldn't), and it gives the following schematic for the NMEA input circuit (see below). This means that your input is indeed properly isolated, like it should be, and you could get away with hooking the +3V3 of your raspberry to nmeain+, and your txd to nmeain-. I'm willing to risk my own st2000+ to check this advice before you attempt it yourself, but you have to wait a bit because it's at the boat at the moment.
Big disclaimer here: this might only work in this particular case and is definitely not reversible: if you try to hook a nmea-out to your pi's rxd this way you are very likely to blow your pi. If you don't understand the electronics that sit behind an input like this, you'd better stick to using safe, pluggable components, like the USB to NMEA0183 module. This might require you to tone down your ambition levels a bit, but they are likely come down anyway after so many blown electronics ;-). But I like your tenacity and don't want to leave you hanging.
Cheers,