Bansci:
I'm glad you found EI parts supplier. Those are hard to find nowadays.
1, The shielding was quite laborious task. I left small gap in the shielding to prevent short circuit. Overlapping may be easier to do, if the overlapped part is insulated.
I put one layer on insulation tape between shield and windings. Insulation of the magnet wire itself may be probably enough, but better safe than sorry. Enamel should withstand the voltage just fine, if not damaged - I was worried that can happen during hand winding.
2, I used both classic "yellow mylar tape" and kapton tape in my transformers, with good results. Mind that mylar tape was made for this application, has known dielectric strength and somehow lower thickness - so perhaps it's preferred. Be careful with insulation thickness, it adds quite a lot to volume consumed on the bobbin. When buying insulation tape, buy rolls with various width, it makes life easier. Cutting narrow slices of a tape from wider one is PITA job.
3, (you had point 2 twice). I did not pot the transformer, though I bought beeswax just for this purpose - it was fairly cheap, under 10EUR/kg at local beekeeper. My intent with potting was reducing transformer hum (by fixing parts that can move against each other in the magnetic field) and somehow improve cooling of the transformer. At the end I decided not to do this step, because mains hum was nearly inaudible (I had to drop a few drops of thin lacquer on outer EI laminations to "glue" them in place - still easy to disassemble if needed). Leaving it as is also let me back door for transformer modification - if any - as potting it makes repairs/modification difficult. Cooling doesn't seem to be a problem with my transformer.
There are also dedicated synthetic transformer varnish solutions (those are usually even more PITA to disassemble). For ideal impregnation vacuum setup is probably needed, which is something I don't have anyway.
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