It sounds like y'all are having 2 issues; 1 problems with account authentication on your phone with your Yahoo account and 2 getting the HTCSync to sync your contacts and calendar information to your phone. Correct???
First let's look at the Yahoo issue,
HTC Inspire 4G Help Thread - Android Forums
Take a look at this thread that discusses Yahoo and issues people have had. You will also see them discussing that there is a Yahoo Mail app. If you are not already, I would suggest you try using it and see if the problem still exists.
This next part does not solve your syncing problem with Outlook rather suggests an alternate way of getting your existing events into Google Calendar. If this is an option you want to try see more information below. If this is not a good option for you I would try contacting HTC support at
HTC - Mobile Phones, Smartphones, Cell Phones, PDAs for help with your HTCSync issues.
First you want to export your calendar from Outlook into a .CSV file. Once you have that file login to your Google account and under calendar and calendar settings try and import the .CSV file. The next steps is to verify all of your events are there. If all the information is there you should be set for over the air syncing using Google Services. But just to be clear using this approach you would need to start using Google Calendar instead of Outlook.
I don't use Outlook or HTCSync so it's hard for me to provide any additional specific assistance.
The last suggestion I would make is, I would suggest you seriously consider using Google services for mail, contacts, and calendar instead of Yahoo and even Outlook. It's a much cleaner solutions using Google services since they have designed the phone's OS to work well with their services. Besides Yahoo has been the cause of several problems with other mobile devices recently.
Besides if you are sharing family calendar's with your husband Google would offer a better solutions for sharing those events you both need to keep in your calendar but don't want to have to enter them twice or email them to one another.
Please let us know if and when you solve this problem or have additional questions.