If I enter into a contract that a previous contract I entered into says cannot be entered, what happens next? What happens if the person of the second 'not allowed' contract sues me for breach of contract? Or if the person of the first contract sues me? Is the second contract nullified? Is the term of the first contract not allowing the second nullified?
If I enter into a contract that a previous contract I entered into says cannot be entered, what happens next? Your question is unclear on whether both contracts are entered by the same parties. Generally speaking, it is valid for parties A and B to enter both contracts unless the formation of the subsequent contract contravenes the rights of another entity who also is a party to the initial contract. If the initial contract only binds A and B, a subsequent contract between A and B is tantamount to mutually agreeing to modify the initial contract. If the initial contract between A and B prohibits entering a contract with C, A's formation of a contract with C constitutes A's breach of the initial contract. The specifics of both contracts and the circumstances would help determining whether the subsequent contract is void and null; whether it is valid at law or in equity for A to have indulged in forming a contract with C (for instance, if B breached the initial contract in a way that frustrates its purpose); or whether compelled performance of the [initial] contract between A and B results in A breaching his contract with C (or in the alternative, whether rescission of the subsequent contract is permissible). If both B and C an "innocent" parties, the court supposedly would look for a solution that preserved their rights to the extent possible, leaving A to carry the cost.
You violated Facebook's Terms of Service, and it's entirely within your contractual agreement with them for them to close your account. You agreed to abide by the contract you "clicked through" when you signed up and opened a Facebook account. They didn't break any laws when you broke your contract with them. You clearly broke the contract by violating the TOS. They are not breaking any laws by not restoring your account. What you see as the value of your personal or business data doesn't really mean anything; it was your choice to use Facebook in the first place, and your mistake to break the TOS. You could try hiring a lawyer to make a case that you see a difference between the personal data you want to retrieve and the business data that caused the violation, but that's up to you; Facebook is under no legal obligation (this could be different, according to your jurisdiction and national laws) to restore any or all access to your account or data, but it's possible they could be persuaded.
What do the contracts with your suppliers and the policy with your insurer say? Changes in government regulation do not ipso facto relieve Parties of the obligations under a contract under common (English) law. Contracts are allowed to allocate the risk of force majeure (and indeed, to define it because it has no common law meaning) but if they don’t, then each party bears their own risk and if they fail to honour their obligations they are in breach of contract. Common law does have the doctrine of frustration, however, that is much narrower and must result in the inability of the contract to be completed at all. And then there are consumer rights which may apply. When the dust settles, we are likely to see a lot of litigation around force majeure. Your venue appears to be complying with both the law and their obligations under the contract so you have no breach of contract claim against them and no trigger for the insurance policy. If you choose to cancel, then you broke the contract. Importantly, the position is reversed in civil law jurisdictions - a party unable to fulfil their obligations under a contract is not in breach.
What's the use of contracts if you can't take a contract violator to court? A clear, thoughtful contract can be a deterrent to misconduct, and Australia is one of many jurisdictions where pro se litigation is allowed. Self-represented litigants certainly have to undergo a steep learning curve and are expected to comply with a code of conduct, procedural law, and so forth, but my point is that hiring a lawyer is not compulsory. Even if for some reason the defrauded party declines to sue the tortfeasor (thereby forfeiting the recovery of the losses), there is a societal obligation to alert others about the tortfeasor's misconduct. Making the contract available to others facilitates alerting them on objective grounds so they don't become the tortfeasor's next victim, and it simultaneously helps for setting the record straight that the damages/losses were not one's own fault. Without a contract, it would be more difficult for others get a sense of whether misconduct occurred at all. Or worst, halfway down the line, you don't have any money left to pay your lawyers, and they leave the case. Hence the importance to litigate in pro per from the start. It is easy for a person retaining a lawyer to postpone (be it due to family obligations, workload, and so forth) his learning of the law. But that postponement only makes the client more vulnerable to his lawyer's subsequent withdrawal when court proceedings are midway: the client would have the dilemma of either finding another lawyer to resume the case --predictably at a higher cost--, or cram the learning curve in trying to keep up with the proceedings.
You would need to know the landlord's reasons for refusal in order to determine their reasonableness or otherwise. If there is a dispute about this (i.e. They think they are reasonable and you don't) you can seek a court order requiring the landlord to accept the sublease or alternatively, that since the landlord has breached the contract, you are entitled to terminate it. Hire a lawyer before you do this.
Do I have any recourse for invalidating all or part of the contract? No. There is a presumption in contract law that when a contract is reduced to writing then what that writing says is what the parties agreed. Also, if you signed it, then you are legally stating: I read it, I understand it and I agree to it - don't sign things you don't understand. If your lawyer has produced something you don't understand then have them redraft it until you do. Would a successful suit against the lawyer for malpractice or negligence make any difference? No. A suit against you lawyer may get you damages from your lawyer but it will not affect the rights of third-parties. What is best practice for avoiding flawed contracts like this in the first place? Read and understand the contract. Educate yourself enough in the law so that you can do this. Your lawyer is there to give you professional advice; you are there to make your own decisions.
Yes They explicitly make the identified illegal act a breach of contract and allow you to sue. Without that, you may have no remedy.
Don't do it. It is of course breach of contract when you signed a contract with no intention to fulfil it. However, you are talking about Germany. German employers take a very dim view of this. While a UK employer would say "good riddance" and do nothing, many German employers would see that as a personal insult. It's something that you just don't do in Germany. There's a good chance that they will do what they can to make you miserable if you do this. For example, inform authorities that you just cancelled your contract which may get a visa cancelled. Or tell the company that you want to start with, which will also take a very dim view of this. Or sue you for damages, not because they want to get the money, but to make you miserable. On the other hand, if they send you a contract, and you sign it, you have a contract.