Absolutely, which is what I know you don't want to hear, since you're probably trying to just grow a couple nice veggies on limited space and budget. But not providing adequate space is, possibly, the #1 mistake I see people make. It's made worse because many gardening lore books/magazines/etc encourage things like what you're doing, growing squash with sunflowers (and other companion/inter planting). And this sounds great! A great companion planting that allows you to maximize your very limited space... but more often than not, it causes two crops to fail, rather than one to succeed. The fact that you got some nice sunflowers and some fruit from your squash is a credit, squeezing that much out of that space means you did well for growing two things there.
But don't be fooled, the squash is weak due to being crowded. Weak plants succumb to disease and insect pests sooner/more completely. Squash are naturally large plants that take up a lot of space. Given perfect conditions, one plant can easily take up hundreds of square feet. They also have very shallow root systems that spread laterally more than down, so they can't tap into subsoil nutrients/moisture as easily if there's no lateral growing room for the roots.
At a minimum (not ideal, but minimum), there should be free growing room in the soil for at least as much lateral space as the plant takes up above-ground. So as far as the plant spreads above-ground, that much space must be provided with loose, fertile soil below-ground for the plant to be healthy. Ideally, another 50%. Adding in competition from the sunflowers, and that plant has a huge lack of room below-ground and is likely root-bound and stunted.
A garden author I like, Steve Solomon (who, I should mention since you also live in the Willamette Valley, wrote the bible on growing vegetables in our region called Growing Vegetables West of the Cascades
- I highly recommend you get a copy and read it, it's where I started), talked about powdery mildew on curcubits (squash and cucumbers), and blames most of the fault for early PMD (powdery mildew disease) on not giving the plants enough space and not giving them enough water. While all curcubits will eventually succumb to PMD at the end of the season as the plant is exhausted from producing so many large, delicious, ripe fruits, early PMD attacking curcubits is usually a result of the plant behind dehydrated. In many cases, the gardener crowds the squash plant too much and as the plant spreads into soil that is crowded with nearby roots of other vegetables, it can't get enough water. PMD starts to appear. The gardener, seeing a fungal disease, incorrectly thinks that there's too much moisture on the plant spreading fungus and waters less, making the problem worse (dehydrated plant = too weak to fight off PMD), and the plant dies.
In your case, I think you're watering the soil, but the water is not reaching the plant. The surface water is being stolen by the sunflowers, and you don't have enough other soil absorbing water to supply the squash plant. The soil under your sidewalk is likely so compacted the squash can't penetrate it very well, so the plant is limited to that small strip and fighting with the larger sunflowers, who are winning.
Here is a picture of my squash plant this year. Some things to note are that this plant gets the entire root zone that it occupies to itself, no competition. Also note, if you look closely at the bottom leaves towards the left, you can see some spots of PMD starting to occur. They haven't spread, or they're spreading so slowly I'm not concerned (note all the lush green foliage). That's because, while all curcubits get PMD eventually, a healthy plant occupying it's own root zone with plenty of fertilizer can fight it off on its own. I'm harvesting more fruit than I can deal with right now from that one plant.
Here is an aerial view showing how much free soil space the squash plant has to occupy:
This is a summer squash, and your's is winter squash, but the same principal applies (as it does to cucumbers, and many other veggies as well).
I hope this is helpful, I know it sucks hearing when you're stuck with little space (believe, I've been there), but it's the reality.