Level 90 is the cap, but I've been on the cap for a while, and I'm not close to ending the game by (I assume) at least a bit. I think I'm pretty close, though. But, that could be because I did a lot more grinding and side quests instead of the main story. I can't say how close you are to the end of the game (or to where I am), since I don't know your main story progress.
5000 coins is the max wallet size (as far as I'm aware), so good job on that. Was I right about those "accessory stones"?
Solarite is decidedly not useless. And you are correct that it is a Metal. I'm sure you can figure out what to use it on. Although if you're really stumped, just ask.
There usually isn't a lot of stuff that happens after a filled bond meter. Some exceptions apply, obviously, like the fan favorite, Nel. She shows up in like 90% of the storylines of all the other characters, and even in the main story.
Well, the game is obviously still a work-in-progress. It's still getting updates, as I'm sure you know. So, the latter half of the game isn't as fleshed out, since as you get to the current-version-end of storylines, and there aren't new areas to explore, stuff obviously gets less novel, since you have less choices on what characters to interact with.
I wouldn't say so. There's no real reason to fight the befriendable characters again and again, unless it's to lose to them. As I said, you only ever really need to beat a character once to get what you need from them. And beating a character doesn't give you more relationship with them (IIRC). You get relationship with Nel via letting her take over, with Violet by wearing her, Domenea by serving her, and Capt. Ceres/Galiene by asking her to come fight monsters with you (You'll level up the affection of both if Ceres is possessing Galiene). Just... try not to carry revival potions when fighting with Ceres, as she will most likely waste them.
No clue who you mean by the "male demon from the Dwarven area". If you mean Dirn Kibhar, I don't recall there ever being a male demon. If you mean that temple in the Furnace Fields, then that guy isn't even a befriendable character. He's just a boss that you have to beat twice specifically to get Inferno Orbs for various stuff. Been a while since I beat him, so I don't remember. He's distinctly not the demon king, though.
And... yes. You can and always could upgrade all your silver stuff to Gold. I can't believe you never thought to try that with all your random Fusing.
Yes, 100 is max alchemy. And no, there isn't a recipe for Greatest Alchemite. You only ever really need 4 of the stuff, and you'll find another spot to grind for it later, so don't worry about it. None of the bosses are really hard enough to need Greatest Alchemite potions. Especially if you actually have proper, non-silver-tier skills, LOL. It's pretty damn impressive that you managed to get this far without 'em.
Mimics spawn randomly, and they can show up anywhere. But, I've heard they have a higher spawn chance if you've gone a while since seeing one. They always give the same base amount of gold, XP, and SP regardless of where you fight them, but their strength scales with the level of the area you find them in, so that's something. There's a total of 6 pieces of cursed equipment in all, and the Enemy Level Up one is the only actually useful one. But, learning the cursed skills does progress you towards more Skill Points, so there's that!
And yeah, Nel does just get to cheat when playing as you. That's the upside for you not being able to control your character and make smart tactical decisions, I guess. That's why she always talks about how your body is "more compatible" with her. She's not just making shit up, I guess. It gets less impactful if you (surprise surprise) have good skills equipped, since you'll have a higher speed and Arcane stat with your skills taken into account.
All in all, you should be having a lot more of a fun time with upgraded skill items. Just, make sure you have the skill for that item learned before you upgrade it, because there's NO WAY to downgrade your items once Fused, and not only will that make you miss out on skills, it'll also make you miss out on Skill Points from learning aforementioned skills.
And feel free to keep asking questions if and when you get 'em. I'll reply, even if I'm not the most punctual about it.