I haven't seen any evidence on "government already has info". It might get all the info if they send a taxpayer into audit. But there's no indication it really knows without audits.
No, the government already has almost all of your information every year from the start. Every time you get a W-2, a copy is sent to the IRS. Same with the vast majority of most tax forms. That why, if you lose any of your forms (at least the ones that say something akin to "This information is being furnished to the Internal Revenue Service."), you can request them from the IRS[1].
Some investment-related returns aren't sent to the IRS but I would estimate that for 90% of people, their taxes could be accurately calculated by the information the IRS has on file.
Additionally, I guarantee that these calculations are being made by the government anyway. If you file a tax return that is mathematically incorrect, you are very likely to receive a correction letter from the IRS[2]. This isn't an audit, it's just a letter saying that your taxes were wrong and they redid them for you, with a new outcome.
The IRS has your 1099s and W-2s. For most people filing your taxes is this weird quiz where you file your taxes and the IRS checks your return against what they already know.
Yes, although it's not always ideal too. E.g. for personal business you're required to conduct all transactions through a dedicated business bank account. But it's easily avoidable by using a different account, and there are cases when you cannot use the business bank account even if you want. But taxes are computed automatically from only that bank account. To stay honest, you need to decline automatic tax calculation and file separate forms to pay more taxes, no one does that.
