I have suggested you to install BI publisher to get the add-on for MS Word installed on your computer so that you can create RTF reports. There is no need for MS Word on the server.

Also, there is no need for a BI publisher on your local computer because it is already installed on a server or cloud. But you will have to install it on your local computer to get the add-on.

After creating RTF files, you would upload them in Apex > Shared Components > Report layout.

You can create a function to check if the supplied customer number exists or not. Below is an example:

Create or Replace function check_customer(p_customer in number)
   Return varchar2 
  is
  cursor c_cust
    is
   select 1 from target_table
    where customer_id = p_customer;
  
   n_exists number;
Begin
    open c_cust;
    fetch c_cust into n_exists;
    close c_cust;
    if nvl(n_exists, 0) = 1 then
       return 'Customer exists.'
    else
       Return null;
    end if;
End;

Now you can use the above function as follows:

Begin
   if check_customer(388383) is not null then
     -- customer exists 
     -- do something here
   else
     -- customer not exists
   end if;
end;

In SQL query:

Select * from sales_orders s
  where check_customer(s.customer_id) is null;

The following are my answers:

1. Yes, you have to create an RTF file in MS Word. Also, install the latest BI publisher version on your Windows, then from the BI publisher windows menu, you will get an option Add-on/Plugin for MS Word. Click on this option to install. After that, open the MS Word, and there you will see the BI publisher tab. Use this tab to create reports.
2. Yes, you will have to create 3 different queries.
3. Yes, you will have to specify/upload that RTF file in the Report Layout.
4. No, there no need to make any changes there.

I will soon write a tutorial on it and then will share it with you.