How does a pinhole form or get left on the weld on a waterpipe?

This can happen as it is so small that the blower can not tell it is there.  You could not really see or feel this.  This can happen when two pieces of glass are joined together badly.  This can happen but a good blower will see the error and then fix it.  Not that even the best let a pinhole go, sometimes it is hard to know.

what is slotting?

This is when the manufacturer or distributor must pay a retailor for shelf space

what is the difference between 50% and 200% markup?

In reality, this is the same thing.  The retail business sector calls keystone a 50% markup.  But the reality is keystone is 2x the wholesale price to make it the retail.  This is a 200% markup.  But when the retailer sells the goods the keep 50% of the money from the item so they call that a 50% markup.  Just wording.

Remember figures do not lie, but liars figure, not that this is any of a lie