Poorly balanced rads may have the effect of reducing the total heating load a little, so yes you may see it begin to modulate when some rooms are still cold. The situation is less dire if you have TRVs since that will compensate for the poor balance to an extent.
To be fair this is all I had on mine, and never found the need to use it. I just left the temp control sat at "2" (where max was "6")
Depends a bit on how the boiler does its modulation... the simplest control would be to modulate so as to attempt to keep the return below the dew point. The flow will be limited by the max water temp stat.
It can only lower it to the minimum of the modulation range. On some boilers this is still quite high power (i.e. 8kW or more)
Probably not... depends on where you are starting from I guess.
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