I think tariffs are simple. Maybe I'll get corrected. They might be closer to a one-time VAT than a sales tax. Just a percentage tax on the value of the imported goods, paid to the federal treasury. Like so many other things they can be used for manipulation and social engineering in addition to the stated purpose of leveling the playing field between USA and foreign workers.
Vacant house seats in a new congress are filled by special election. Period. No appointments. Rules for vacant Senate seats vary by state. In Florida and some other states the governor appoints the replacement, followed by an election that could be a couple years off.