Yes, Hotel rooms are hard to come by during Octoberfest, especially the weekends. Trying to secure something early might help, the other strategy people use is to stay outside of town, along an easy rail trip. You might consider Freising, Tegernsee, or Augsburg, if you search on this forum, several conversations about places to stay will come up.
The opening weekend is nice, good parade, there will be crowds, but within Munich they are manageable, at the fest grounds, a bit more intense.
You probably will not be purchasing a reservation in the tents. A reservation requires you to pay up front for food and beer for 10 people at a table. As a couple, you have several options. The most common is to just show up. It costs nothing to get into the tent, you can go in, look around (I suggest at least going in to look at as many you can stand), however, you have to be seated to order a beer or food. All tables have a reservation card for the day, if the table is open, ask the wait staff, you likely can sit down, up to the time that the table is reserved for. If you are a gregarious type of person and see a table with 4 or 6, you might inquire if you can sit, be good guests and converse and sing with your table mates, if they reserved the table, maybe offer to buy the host a beer. The other option would be to find some type of tour, might cost you a bit more, but if you want to guarantee a seat at night, that might be worth it.
Obviously picking up a couple seats can be difficult on opening weekend, but walk around, you never know. Evenings during the week can be tough, but not impossible, I have had tremendous luck going the Monday after opening weekend, especially in the afternoon, having a beer in multiple tents. While having fun with a group is the main attraction, if you are a bit outgoing, you can quickly become part of the group, even just the two of you singing along to the bands will be great fun...learn the lyrics to "Ein Prosit" and The Fliegerlied or 'So ein Schöner Tag'...and you will fit right in (or don't learn them and fake it, you will be fine).
As for other fests, less than a week after Octoberfest opens, The Cannstater Volksfest in Stuttgart opens, more what Oktoberfest used to be, now getting almost as many people, but fewer tourists. It is a great time as well. While other towns have fests, nearly all are well before or after. Further afield would be some wine festivals.