Do Chansey swarms even exist in Heartgold?
I've done the soft reset trick like 200 hundred times and it's always either:
Will there ever be a Chansey swarm?
and if they do, how often?
Chansey can found in:
Referring to your edit: it is randomnized.
Chansey can indeed be found in HG in Route 13,14,15 and the safari zone. Thats it.
Route 13, there is a swarm. 40% encounter rate.Otherwise, you have a 1% chance of finding it in the grass. Yes, 1%.
Bulbapedia/ Route 13
Route 14 & 15 : 1% encounter rate.