Can I pre-order and buy / ship Xbox Series X from my microsoft account as a gift for...

No guarantee, but baring shipping issues they are pretty good at getting it there on time, if not early.

I pre-ordered the Series X direct from Microsoft, and it appears on my MS Order History. However, I've not received a confirmation e-mail that the order was placed. Was there a glitch with sending of e-mail confirmations for the pre-orders? Should I be
concerned with the status of my order?

The only way you can get it is if you pre-ordered from Microsoft Store. I pre-ordered and they tried to upgrade my shipping, but that got all screwed up and I got a code to get one free game from Microsoft Store including shipping and I chose Gears 3 Limited
Edition. The car isn't in DLCs yet and I don't think they will. Maybe you will get lucky and someone will gift you one.

I have been buying Microsoft gift cards as a way to save up to buy a Series X. When the time to pre-order came around, I had $550 in Microsoft store credit. That was more than enough to cover the cost of the console and taxes. I finally got an order to go
through at the Microsoft store, but it did not use any of my store credit and it is going to charge my debit card instead. I have contacted support but was told the only way i can use my store credit is to cancel the order and re-order. Obviously if i do that,
i will not get the Series X.

This means I will probably be cancelling and will not get the Series X at launch. After already spending $550 towards it, I’m not going to spend that much again because I cannot change payment on the order to store credit. I can change cards, change it to
Paypal or even mobile billing but not store credit that is on my account. That doesnt even make sense to me.

As stated above, funds from Xbox gift cards are added to your Microsoft account balance. They can be used to buy games/subscriptions on the 360, One, Series S/X, Windows, and you can use them in the Microsoft Store.

The point of it is for example: right now on the Microsoft Store you can get 10$ Xbox gift card when you pre-order titanfall. When you try to pre-order it assumes you want the disc shipped to your house.