1. Belgium.
2. Online poker is allowed, any player can deposit and play on a poker site without any problems. Although some banks might not allow you to deposit on gambling sites, so you'd have to use an e-wallet like Neteller. Even if the bank finds out, they don't care as long as you don't deposit directly onto a poker site.
3. Now that's a goddamn jurisdictional nightmare. If you want to withdraw your money onto a Belgian bank account, you're obliged to pay 30-33% of your winnings as taxes. This is the official rule for professional poker players in particular, and for gamblers in general.
Although if you keep your withdrawals small (below 1000 EUR) they usually don't even notice. If the local IRS catches you, you tell them your money comes from an e-wallet, and since it's not a national company, they have no right to disclose your personal info, and IRS won't find out you won your money playing poker. You still might have to pay taxes on that money, so it's best to stay below the radar.
4. If you work in a country, you're taxed by that country, that's the local policy. If you play poker for a living though, and don't have a Belgian bank account or don't use it to put your winnings on, then the money's all yours.
5. An association of card players is now trying to negotiate with the authorities to legalize poker cash games in Belgium. Right now home games for money are illegal and can cost the organizer (not the players!) a serious fine. Cash games in the casino's are not allowed, only tournaments, and they even have to request a license every time they want to organize one. In general, not a very poker-friendly country.