Is kettlebell swing an exercise that you can, with enough training to adapt, do for hours, just like you can row or swim for hours?
Yes. Though obviously there's an individual upper limit on how heavy the Kettlebell could be, as their total power output (the amount of work put into each swing multiplied by the frequency with which they're performing the swings) would need to be within what their aerobic energy system can provide.
Is it a viable workout to keep up the form in the winter, or it will have a negative impact on the spine?
Swinging a light kettlebell is a viable form of cardiovascular exercise, if that's what you're asking.
I know that working in the leaned position is bad for the spine
That's actually very unlikely to be true. See:
Wai, E. K., Roffey, D. M., Bishop, P., Kwon, B. K., & Dagenais, S. (2010). Causal assessment of occupational lifting and low back pain: results of a systematic review. The Spine Journal, 10(6), 554–566. https://doi.org/10.1016/j.spinee.2010.03.033
Stacy A. Clemes, Cheryl O. Haslam, Roger A. Haslam, What constitutes effective manual handling training? A systematic review, Occupational Medicine, Volume 60, Issue 2, March 2010, Pages 101–107, https://doi.org/10.1093/occmed/kqp127