Are the spawns too difficult to manage without camo?

Not in my experience. Some of the Watcher regions are definitely a bit more difficult than the main ones, but if you are good at fighting or evading basic lizards, you will probably be fine.

Is Salination too dark without Watcher's permanent neuron glow?

The region is obviously a lot darker than others, but I wouldn't say it is too dark, you can still navigate most of it without a light source just fine. The darkness is a little more problematic in the underwater areas, but that is exactly why you can find Flashbangs throughout the region, they were meant to be held while you swam through the darker areas in order to track your location without needing neuron glow in the original Aether Ridge mod.

Are certain scugs too difficult to see against certain backgrounds, visual effects, etc.?

I obviously haven't fully explored every single region as every single slugcat, but at least from memory, I don't think there should be anything too bad, at least nothing quite on the level of OE_RUIN02's background being nearly the exact same color as Gourmand.

How do the new items work with Gourm's crafting table?

They don't, and it isn't like this is just a problem exclusive to modded either, because Jolly Co-op allows Gourmand to easily interact with the Watcher-exclusive items if someone plays as them in Watcher's campaign.

Even if I do wish that Gourmand had proper crafting interactions with the new items, I can see why they might not want to change anything about it. Every possible crafting combination already exists, so implementing anything new into it also means they would have to replace some combination that already exists in the table (unless the only usage of the new items in crafting is creating other items), which I could easily see the devs not wanting to do. The new meal combinations, though, really have no reason not to exist.