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Build the hill and plant the seedlings. You're running tight for time to get mature pumpkins, although some of the new varieties have a shorter maturity date.

You can prune your pumpkin vines, however if these are one of the smaller, Halloween type you can also grow them on a sturdy trellis. Just use knee highs or old, clean pantyhose as a "sling" to hold the growing fruit. It will get you odd looks and occasional comments from your neighbors. I grow sugar baby watermelons this way. The fruit is clean and I have little problem with disease or insects. However,here is an article that gives more information on pruning pumpkin vines. And a just in case ^-^ article on ripening green pumpkins
http://www.informeddemocracy.com/pumpkin/growing.html

https://www.gardeningknowhow.com/edible/vegetables/pumpkin/getting-green-pumpkins-to-turn-orange-after-the-pumpkin-vine-dies.htm

This could be due to poor pollination. Poor pollination affects fruit set. High temperatures will cause plant stress. As a result, any pollinated fruit may have simply aborted. A heat wave can also deter bees from their job. You could also have more male flowers than females, which also inhibits pollination. You could try hand-pollinating your pumpkins, which is done in the same manner as with squash. Here is an article that may help: https://www.gardeningknowhow.com/edible/vegetables/squash/pollinate-squash-by-hand.htm

Here is an article or two that you may find helpful:
https://www.gardeningknowhow.com/edible/vegetables/pumpkin/pumpkin-growing-tips-for-halloween-pumpkins.htm
https://www.gardeningknowhow.com/edible/vegetables/pumpkin/how-to-tell-when-pumpkins-are-ripe.htm

Some yellowing of leaves is normal and since the rest of the plant looks healthy, it would not concern me.

I hope this helps.
http://www.clemson.edu/extension/hgic/plants/vegetables/gardening/hgic1256.html