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Hello

1) Subletting is forbidden by default, except if you have the agreement of the landlord.
In Wallonia, it must be written on a document and signed (which I would always recommend to avoid any problem in the future)

2) The sublet must set his main residence in that accommodation.
The rental period can't exceed your own rental period.
You must inform the sublet that this is actually a subletting and you must inform him about his limited legal rights on the rental.
If the landlord cancels the rental agreement, you must inform the sublet within 15 days that the rental agreement will be cancel at the same time as your agreement with the landlord.
If you cancel the rental agreement, the sublet will have a 3 month period in order to leave, but you'll also have to pay him 3 months of rental price.
The sublet can't sublet his rental agreement. No exception allowed.
Legally speaking, the main tenant remains solely responsible for the payment of the rent, the maintenance of the building, and the return of the accommodation at the end of the lease, even if the sub-tenant does not respect his commitments.
You will also be responsible towards the sub-tenant if you could not peacefully enjoy the accommodation.

Let's be honest, if there's any problem, it would always be your fault and you'd have to pay.
That's one of the reason it's not super popular in Belgium~

3) Yes, as long as your landlord agrees.