You may get different points of view on this, this is mine;

1) Some people do! but think of this; you remove all but one, plant it and on backfilling it is knocked off.........result the chitting was a waste of time.

I have found it makes little or no difference.

These sprouts / chits are individual haulms (tops), not roots as you suggest.

The original seed potato rots away, and roots develop from the haulms and the new potatoes are swellings (tubers) that grow on these roots.

This is one of the reasons for 'earthing up',that is, it encourages more roots, and the potential for many more potatoes.

It might then be said that this area of soil will not sustain lots of potatoes so it is better to reduce the number of haulms and get few but bigger potatoes.

So really, reducing the chits to one, a few or just leaving them all on is just a matter of personal choice.

Regarding the length of these; this does't really matter the most important thing is the state of the soil, i.e. it is not water logged or frozen.

In fact you can plant seed potatoes without chitting, by not chitting, this only means that the haulms will take a little longer to emerge.

If you plant out too early the new emerging growth might be nipped by late frosts so this sets you back and the benefits of early planting has been lost.

I tend to work backwards from when I expect the last frosts.(have a look at 'Frost Zones on my website)

I find in my area it takes around three to four weeks for the tops to emerge so I plant out about three to four weeks before the last anticipated frost, assuming the ground is in a fit state to do so.

I think patience and local knowledge is the answer to much of what you ask.

If you look in my website you will see it is a calendar of events brought about from the days I was in your positions i.e. each year I kept notes on what I did and the results, then the following year I adjusted my tasks accordingly.

I always err on the side of safety a sow/plant later if I am in doubt and generally I find that through the resilience of plant life (not me) I get a good a crop of what ever I am growing.

I hope this answers your question.