Without a dedicated return line, the only one that lets you adjust the amount of warm water that goes into the cold line used as the return, is the RedyTemp. All of the others can't be adjusted, or at least easily in their cutoff point.
Depends on how your house is layed out how easy it would be to add a recirculation system. Most of them can be setup to to only come on upon user initiation.
My guess is that if used in this demand mode, rather than a continuous or timer based use, the thing wouldn't impact the tankless' operational life - it would just look like a longer demand cycle.
I have a RedyTemp unit hooked up so it only brings the water at the vanity to barely warm. The shower tap off that line is closer to the WH. So, after flushing the toilet, it has purged all of the warm water out of the line. Your results may differ because of your pipe layeout.
If you use a boiler for heat, many of them with the use of an indirect water tank, would end up being less expensive to install and maintain than a tankless. The better tanks lose only about 1/4-degree per hour in standby.