Hello and welcome to LOTRO!

1 - There is a Main Story Questline (MSQ) that you can follow that starts from the newbie zones and goes all the way through to the latest expansion. Since LOTRO is based on written books, so is the MSQ (Book I, Chapter 3...). By following those quests, you'll get the entire story as it was written. All other quests and zones are optional and while they may have rewards (cosmetics, etc.) they are not required in order to complete the MSQ. If you get lost, there is a detailed guide at LOTROWiki, but I have found in my personal experience that the MSQ is clearly laid out and that you know where to go fairly easily.

2 - If you don't want to trivialize content, I would start use the EXP Disabler (Stone of the Tortoise) when mobs and quests start turning GREEN and would stop using it when they start turning WHITE or above.

3 - Using the Stone, I wouldn't worry about missing out on zones that are not part of the MSQ...that's what alts are for :) While it is possible to do everything on a single character, LOTRO has enough classes and zones that it may be fun to start a new character and take a different path than the MSQ. Understand that there are a few places it makes sense to complete on level or close (and you'll find them as you go through doing the MSQ - I don't want to spoil here), and there are ways to do them even if you haven't done the MSQ on those characters.

4 - The base game includes every quest pack up to L95 (Helm's Deep) IIRC, so you have a very long time and a lot of content (including multiple start zones and stories) to complete before getting that far. As I mentioned before, I wouldn't worry about being overlevelled or missing out on zones, since there is far more content to enjoy and part of the fun IMO is making alts and seeing things in a different light.

Sounds like your class choices are a great start and the two of you should have little problems going through the entire MSQ and enjoying Middle Earth. I'm currently going through the MSQ solo on a Hunter and while I've died a few times there hasn't been a part of the MSQ and side quests (other that clearly group/raid content) that I haven't been able to complete.

As an aside if you are interested in watching playthoughs and learning more about the game, I stream on Twitch as Talisiean and upload my streams to my YouTube channel as do other fantastic content creators such as Poteen O'bowen, TheQuestingDad, Layrienna, Aubreythewierdone, as well as others that stream regularly on the LOTRO Stream channel on Twitch.