This blockchain is deletable. Does this mean if I check for my Pascal coin buy transaction after 2-3 months on Polo, it wont show up? I hope that wont be an issue. In that case I will immediately store the CSV.
Deletable blockchain is a feature, its not and obligation

people can keep all history of chain like merchants

That seems to be correct. I was able to restore full blockchain on the Linux wallet recently. It has the full history.
It is possible to run it without the blockchain with only safeboxes.