Websites explaining produce seasons totally exist. I don't know where you live, but I'm sure you can find the info.

If you leave soapy water standing in your dishes, the soap should cut down on slime growing on them.

Spinach is available canned! Get it that way and season doesn't matter.

Kale is not like cabbage. It's a leafy green, sort of like turnip greens or mustard greens.

Celeriac is good roasted and then put in soup. Find smaller roots of it, and when you cut the ugly off, cut a little deeper; if you don't put much in the soup besides celeriac you should be able to eat it in as little as two sittings.

Turnips don't look anything like carrots; do you mean parsnips? Wikipedia says you can eat parsnips raw.

Yellow squash comes small. I usually have it with fish, and use half of one in a meal; if I'm having only squash for a meal I can definitely eat a whole one. I like to slice them up thin and then roast them (line pan with foil, coat with olive oil, pop in the oven at 425º for 15-20 minutes).

Leeks are not as much food as they look like. You can roast them (it's wise to parboil first though), or put them in soup.

Broccoli is amazing roasted, in soup, or with pasta. Cauliflower is good in soups, curries, or casseroles. You really need to find out why your fridge won't let you keep leftovers for a normal amount of time and fix that; I can easily eat a batch of cream of cauliflower soup that contains an entire head of cauliflower over a few days, but if you can't keep soup around for a few days you may be in trouble. Cauliflower and broccoli are also edible raw, optionally with dip.

Many but perhaps not most vegetables go well with others. My brain isn't generating a useful algorithm here but I can give you my best guess/experience regarding any combinations you think look likely.

There is a lot of information available about substitutions online. I am an improvisational cook and can probably help you out if you're missing something for a recipe (I have good instincts about when you can leave it out, when you can replace it with something else, and when you can't make the recipe at all without the ingredient).