Offline led driver (15W) with bifilar wound flybck transformer is OK?

Hello,
This product does not have the regulatory 8mm of isolation spacing from the mains side to the LED side....

....At time 20:52 of the following, appears to be the proof that this mains light bulb not isolated....
https://www.youtube.com/watch?v=YLlpoZaBx-g

Therefore, is it ok if i do a 15W flyback with bifilar wound enamelled copper wire (not triple insulated copper wire).
In a typical offline flyback, the 3500VAC regulatory isolation spec means that the transformer has a high leakage term, and 10% of the circuit losses are due to the leakage inductance.....if we can bifilar wind with enamelled copper wire then the leakage will be far less, and so too will be the circuit losses.

Will a bifilar wound, 1:1 flyback transformer, have the lowest leakage possible for a flyback transformer?

You are right, bifilar windings have the lowest leakage inductance. However, they aren't used in most offline applications due to safety and EMI concerns. If you want to use a bifilar transformer, you certainly can--as long as you don't care about safety.

If you're trying to make a product that will be sold, you will need to conform to some safety specifications, such as UL60950-1. Then you'll have more leakage inductance and reduced efficiency. To get the efficiency back, you can use active clamps, or switch to a topology which inherently recycles the leakage energy--but it isn't cheap to do that.