The car is actually a stock Polo G40...

Currently, i am remaping my ecu and i would like someone to reply in my question.
The car i a VW Polo G40 with a digi 1 ecu

Leaning the AFR from 15-15.8 at low RPM (till 3500) and low load till 80 absolute pressure, and increasing a few degrees timing, can damage the engine?

When i lean the AFR i mention a lack of power and lack of throttle response, but when i add ignition timing this hesitation almost goes....
I have added about 6 degrees of ignition and i haven't mentioned any knock. I will add a few more degrees of ignition, in the next few days, but, i don't know the limits of a stock 250.000km engine with use of 95oct fuel...

Lets say, i detect knok at x degrees. How much, have to pull back to be safe?

Also, i am going to have a big trip of 1500km. Is this lean AFR condition damage ane engine part?

Leaning and advancing without egt and knock sensing, just before long trip looks like bad idea to me. If you want to improve you mileage, don't lean it out to misfire, lose power or to melt something. Six degrees might be too much advance to add, but it depends on fuel quality, CR, AFR, IAT and camshaft type and angle

Its a stock engine with 8.0:1CR, ignition is way to retarded.

How can i improve mpg?
Whats the steps?