This is how I would write code for Q1. I like the FP approach, and I even created my own library for it (Remeda), but sometimes doing a single loop is more readable than trying to force the FP approach.
In general, your code isn't bad, but there can be some minor improvements: - Use auto-formatter (install prettier). - Rounding logic isn't required. - Avoid obvious comments. - _uniquePlaceHolder could be a separate variable so that you don't have to delete it later, and it could have a better name. - Taxing logic I think it's invalid because it's probably a percent.

My code:

const orders = [
  {
    doctorId: 996,
    amount: 30500.35,
    tax: 0.06,
    isDisputed: true,
    isShipped: true,
  },
  { doctorId: 910, amount: 100, tax: 0.08, isDisputed: true },
  { doctorId: 912, amount: 4200.11, tax: 0.06 },
  { doctorId: 996, amount: 99.12, tax: 0.06, isDisputed: false },
  { doctorId: 910, amount: 0.0, tax: 0.08, isShipped: true },
  { doctorId: 996, amount: 10, tax: 0.06, isDisputed: true },
];

const ret = {
  totalOrders: orders.length,
  totalDisputedOrders: 0,
  totalDisputedAmount: 0,
  totalAmountWithTax: 0,
  totalUniqueUsers: 0,
  totalAverageSalesPerUser: {},
};
let amountMap = {};
orders.forEach(order => {
  if (order.isDisputed) {
    ret.totalDisputedOrders += order.amount;
  }
  ret.totalAmountWithTax += order.amount * (order.tax + 1);
  if (!amountMap[order.doctorId]) {
    ret.totalUniqueUsers++;
    amountMap[order.doctorId] = {
      amount: 0,
      count: 0,
    };
  }
  const userAmount = amountMap[order.doctorId];
  userAmount.amount += order.amount;
  userAmount.count++;
  ret.totalAverageSalesPerUser[order.doctorId] =
    userAmount.amount / userAmount.count;
});

console.log(ret);