For cover crops, aka Green Manures this is what I Would suggest:
Especially if you have a shorter growing season, buckwheat is the way to go. It also is not difficult to mow down/till (at least if you have power tools)
Clover is not bad, neither are any sort of bean crops.
If you want to get some use from your plot, go to beans, either dry or fresh beans/peas. They will do the same things, and give you something to eat, either fresh or dry.
Alfalfa ...
Well When I lived on the farm it as a perennial that gave 4 years or so of solid preformance, and even with farm implements was difficult to eradicate completely. Something you wanted for a forage production crop, not something that you wanted for a home garden. I would stay clear away from it. In addition it is a slower developing crop, where beans/buckwheat/clovers/peas can be expected to "harvest" the first year of planting, alfalfa was not expected to be fully ready till the following year.
What I have done and my father has done very successfully was tilling the soil, then planting buckwheat right away. Let the buckwheat get to the point of nearly dieing back, mow it down and till it all under. this was accomplished in august after a later spring sow.
The green material would then have some additional time to decompose that year and the next spring the soil was ready to go with the typical addition of some compost.