the reall money,
Great questions and smart move to be using the C2 unit! The reason for the higher HR on the rower than the bicycle is because you use more muscles when you row. I use the C2 units with ALL of my clients - and I don't get them for free! There simply isn't a better way to work full body - moto specific for sure.
Regarding your LTHR, in addition to Tim's suggestion of the 3 x 8 minutes, you can also use a pre-programmed workout in the C2 unit 500 Meter Intervals with 1 Minute rest. You can capture and calculate exactly as Tim suggested, but it also allows you to capture what MotoE refers to as a Power Calculation of: Distance (500M); Elapsed Time; Average Heart Rate; Maximum Heart Rate - all at a set load level.
If you complete 5 x 500 Meters at a specific load level (say damper setting of 5), you can evaluate your muscular strength, muscular endurance and lactate tolerance each time you assess (ideally every 5 to 6 weeks depending what time of the year you are completing these assessments). If you would like a simple spreadsheet that you can document all of this information, please feel free to email me at email@example.com
and I will send you a copy and how to interpret the data.
Keep your questions coming on RacerX VT!