Any question answered...

Can you make aluminium welds as neat a steal ones, and how do they "fill" the joins on Alu frames, like Bianchi Pista Concepts?

Body filler.

Not necessarily.

Or grind them down afterwards.

Someone with a TIG and some talent can make beautiful even "stack of coins" welds, as opposed to the welds on my Fuji for example.

Yep, this.