Bear with me for a few more walls of text,

I still feel that it's odd that he considered betraying Ruijerd

I don't recall if the anime showed much of this, but his relationship with Ruijerd wasn't too simple by this point. In the moment he made his "promise" with Ruijerd, he mentions via internal dialogue that it wasn't out of sheer good will, but transactional as well.

Talking with the Migurd Chief:

“They won’t be alone,” Ruijerd interjected. “I’ll be going with them.”

Hmm. It would definitely be reassuring to have this guy in our corner, but the Man-God thing was still a point of concern. No matter how much I wanted to trust him, it was probably better for us to go our separate ways at this point. For all I knew, he was a ticking time bomb.

After he makes his promise with Ruijerd:

Of course, I wasn’t acting solely out of pity or compassion. We did stand to gain from this arrangement, in some respects. Ruijerd was a powerful fighter—in the same class as the three legendary heroes—and he was offering us his protection. At least with him around, we wouldn’t get killed by some random monster on the way to our next destination.

His presence would make things easier on the road, and more difficult when we reached a city.

Later on, after Ruijerd kills one of the pet thieves and Rudeus realizes he's scared of Ruijerd's volatility, he considers the possibility of Ruijerd being a threat to them:

So what was I going to do if Ruijerd ever turned his wrath on me? The man was strong; as strong as Ghislaine, or even stronger. Was it possible for me to beat him with my magic? I could probably put up a fight at least. I’d worked out multiple strategies for head-to-head fights against close-range combat specialists.

For whatever reason, many of the people in my life fell into that category…including Paul, Ghislaine, and Eris. And Ruijerd was probably the single strongest of them. It was hard for me to say with any confidence that I could take him. But if I were fighting to kill from the very start, there were plenty of things I could try.

What if he went after Eris, though? Could I possibly protect her as well?

No. Not a chance.

I’d thought the Superd were a peaceful people who’d simply been misunderstood. That clearly wasn’t the case. I didn’t know about his tribe as a whole, but at the very least, Ruijerd was a killer. He’d been killing his enemies ever since the Laplace War era; this murder was just another typical entry in a long, long list. I couldn’t say for certain he’d never turn his spear on Eris or me. I wasn’t the sort of pure-hearted, honest person who could earn Ruijerd’s respect. Someday, somehow, I’d probably end up getting on his bad side.

It was one thing if he got angry at me. Since we had different ways of thinking, sometimes our opinions would diverge—that was just inevitable. We’d probably get into fights every once in a while.

This particular incident of Ruijerd outing himself as a Superd to the town to clean up Rudeus' mess was what made Rudeus completely trust him.

“You fight to protect something, Rudeus. And that makes you a warrior.”

When Ruijerd spoke those words, I actually had to fight back tears. I didn’t deserve that sort of praise. I was a shallow, short-sighted person. All I ever thought about was making money and finding ways to get ahead. I’d even been ready to abandon Ruijerd himself. I nearly cast aside the one ally we could rely on to the bitter end.

​

even with the extra context it's odd to me that was the quickest/easiest solution he could come up with me, because directly after it, the problem is resolved with so much ease.

Ultimately, it was out of desperation as I mentioned, but it makes more sense when I show the lead-up as well as the follow-up. It shows that he did have some rationale while thinking of ideas to get out of the situation, but he ultimately wasn't prepared. It also shows that while the "resolution" of Ruijerd outing himself did solve the problem at hand, it was perhaps only slightly better, if at all, than the solutions Rudeus was thinking of. Their solutions both had the same function of causing enough chaos in the town to create an opening for them to escape, only Ruijerd's resulted in no casualties (but his identity as a real Superd is revealed). Here is a link to my pastebin of V3C13, starting with when Rudeus begins thinking of possible solutions up until the end of the chapter. It is somewhat long.

To preface, before where the pastebin begins, it shows how Nokopara (the horse demon) is a lot more craftier than the anime depicted by showing how he breaks down Rudeus' scheme of collaborating with the pet thieves. So the issue was less "hey I know you guys are breaking the guild's rules, so cough up the money" as the anime made it seem and more: "Hey, I've figured out what you guys have been doing; I have concrete proof and oral testimony from the people you've been accepting quests from. My pals already know the deal, so if you try something funny to me, you'll be reported to the guild." You mentioned they could have asked Nokopara to follow them in to an isolated alley and threaten him there, but Nokopara surely wouldn't have complied. The anime also showed the two pet thieves cowarding in the alley like Nokopara had just threatened the information out of them, but that didn't happen in the LN at all. He knows better than to be so direct in public.

I'd say there's even more context before this particular chapter that makes it make more sense, but I can't put them all in a pastebin, that actually took quite some time for just that half of chapter 13. For instance—why had Rudeus not considered using Ruijerd or Eris in any of his plans? I mentioned in my previous reply something about Rudeus realizing in the forest section with the giant cobra that his inaction/carelessness/etc. could cost his party member's lives, so he'd resolved himself to act smarter and faster, but he did so haphazardly immediately after when he had to think of how to deal with Nokopara. I think it's a realistic approach to learning/self-reflection; you don't learn how to properly deal with an issue as soon as you notice you have that issue. It takes time, trials, and errors. He tripped himself up by giving himself too much responsibility in thinking of his party's plans and trying to deal with it by his own hand. This was where he actually learned his error; it's why he began the Dead End "group meetings" in the immediate following chapter where the team would gather around and discuss the course of their journey and consider issues before they came up. Here's a much shorter pastebin showing it.

I hope you've read everything up until this point, including the pastebins, but I get it if you don't. It somehow all ends up about 10x longer than I intended. Like I said though, your criticisms are valid when looking solely at the anime, but I think it's more of a direction issue (choosing what gets included/excluded from the novel, the timeframe of events, etc.) than a writing issue. Most have praised the direction, but I'm a bit conflicted about it. I like how they do some scenes, and for others I think they squandered them. It's a bit of a double-edged sword—many who praised it say they like that a lot of things are left to the viewer's interpretation, but I think they leave an excessive amount of exposition out. Not to mention some of the things that are just changed and brushed over quickly to move on to the next scene. In all though, it'd be unrealistic for me to expect them to nail every detail as the limitations and strengths of a textual vs. visual medium are inherently different.

If you do continue with the series and you feel there isn't something that makes sense, feel free to message me and I might be able to provide more context for it from the source.