Are fuel tank modifications safe?

I have a friend who purchased a Fiat Doblo which had already been adapted into a WAV. The modification included a lowered fuel tank which now protrudes c.3inches below the visible body and, when driving...

Is it better to top up fuel at half a tank or to refuel when almost on reserve?

Is it better in the long run to top up fuel at half tank on a regular basis or to refuel when almost on reserve?

Can I syphon the diesel out of my car?

Could you please advise me how to get diesel out of the tank of my car that is possibly going to be written off? There is £40 worth in and we've tried to syphon it out but there is a something stopping...