Could the President remain in office after an impeachment? Ask again in 2-3 years, but today, no. He'd be removed, no federal employee (civilian or military) would continue to take orders from him except perhaps his Cabinet and the DOGE boys. They all swear an oath to the Constitution, not to the man. In 2-3 years, though, could be a different culture.
Anyone who imports and is assessed a tariff would have personal standing to sue, arguing that the tax is illegal. I think there are some procedural requirements first to e.g. formally complain, but the dispute would eventually get its day in court.
However, these tariffs are being applied at least under a thin veneer of law regarding declared emergencies. It's clearly beyond the intent of the law for Trump to do this, but it's hard to draw a bright line between this and (e.g.) ambitious environmental regulation.
Some of the senators are breaking away though.
If the country survives this though the next thing through executive order could be just as bad or worse. There doesn't seem to be any realistic way out of this insanity.