1) The main consideration for height gain is the kind of plants being grown (whether the freebie seeds produce Indica or Sativa-influenced plant), and sometimes the age of the plant (seed plants flowered young can shoot up more than mature plants).
Assuming that the plants will double or triple their vegetated height is a pretty safe bet in most cases, but remember that strong influences from either side of the family could still cause Sativa plants to gain more height or Indicas to end up smaller than planned.
2-3) Trimming the main stem won’t slow the rate of growth, though it will reduce the height of the plant, of course. If anything, pruning can cause a very slight increase in the rate of growth, as the replacement shoots rush to catch up and become stems.
If the replacement stems are given adequate time to re-grow (at least 2-3 weeks before flowering), they can produce as much (or occasionally even more) than a single stem plant. Indica/Sativa makeup plays a part here too, as a decent Sativa influence will often encourage the replacement stems to grow well and fill up with long colas.
4) A little, but if it’s done carefully they respond well. As long as the grower doesn’t lop off too much foliage at one time (no more than 30% is a good rule of thumb), healthy plants seem to react to pruning as a challenge and grow with vigour.