Would probably work. Maybe. The Lowrance manual says you need "fast heading" data. I don't know what they mean by "fast" or whether Raymarine meets that rate without the gyro rate compass in the course computer. (My course computer manual says it puts out "fast heading data"). Considering you're mixing vendors I suspect the only way you'll know for sure is to try it.

The other question I have is whether "fast heading data" is required for it to work at all, or whether it will still work with slow heading data (but maybe just not track as accurately in rougher conditions).

Yes you need a terminator at each end, and the connections to instruments (and between the Seatalk NG and NMEA2k) come in as "drops" off the network. Even if your network us just a string of T connectors, you need the terminator at each end. Search the web for: NMEA 2000 "network guidelines" or NMEA 2000 "topology".

Edit: One more thing -- you may need an extra Lowrance NMEA2k T connection to allow the 3rd connection: Power, Seatalk, HDS. Considering the Seatalk powers the buss you might not need the NMEA2k power connection, but it's better to do it "by the book".