Hello
I'm not an english native speaker but i'll try to help you anyway. I would prefer that a native speaker confirms what I say, or corrects if i did something wrong (in which case i would be really sorry!)
For the first exercise :
"Unfortunately we heard no more from her, so we've had to start advertising again. "
I think the two tenses used don't fit together. I would say we heard no more from her, so we had to start....
The job's being done done by Mr Brown's son, who shouldn't be working really because he's unhelpful and sometimes he should be quite rude to visitors.
I guess you speak french because you live in Belgium, so i'm gonna explain in french, if you don't, then i'll try to explain in english.
Si tu écris
"who shouldn't be working because he's unhelpful" ca veut dire qu'il ne devrait pas travailler parce qu'il ne sert à rien.
Si que le sens que tu veux donner à la phrase c'est qu'il ne travaille pas vraiment parce qu'il n'est pas d'une grande utilité... :
He's not really working because he's unhelpful ...
Si tu dis he should be quite rude to visitors tu l'encourage à etre odieux à l'encontre des gens! Il vaut mieux dire sometimes
he is quite rude to visitors.