My personal experience is that the only benefits to your lawn (and specifically the grass) are it will fill in bare/thin spots which will help with water retention in times of drought, and if you discharge your clippings it will feed the lawn through decomposition (this is probably it's biggest asset and answers your original question [plant is still alive but the cut portions will feed the soil]). But that's about it. Great for bees, great for having a green "lawn" without much work and little/no fertilizer, but it will crowd out grass if there are soil issues (compaction, lack of nutrients, etc.), and I personally don't like the look during flowering season.

Everything I have seen over the years tells me that the nodules of Nitrogen that legumes (clover) fix on their roots is not available until the plant either dies out or goes dormant.

So I take it you're not a proponent of clover lawns?

Do not misinterpret what I wrote. I have been an advocate for White Dutch Clover in lawns for years, a common addition to grass seed when I was growing up until Dow Chemical developed a plant poison that killed it.

The all important question about clover is: does clover share the nitrogen it fixes with nearby plants while it is still alive?

1.) If you mow it, it does.

2.) If it doesn't share, what is the life expectancy of a clover?

Depending on where you are, don't many clovers grow as annuals? If so, you're getting nitrogen released into the soil every year as the plants freeeze and die. (Here in central Texas, it seems to start in late winter, then dies off in the heat of summer.)