What were the French trying to do at Dien Bien Phu? I'm very unconvinced that even had the French force not been destroyed that they could have accomplished anything at all.

In very broad strokes you're correct. The french wanted to entice the Viet Minh to engage in a pitched battle where they hoped their air superiority and artillery would give them a decisive advantage.

Seen from a bigger picture, the French were well aware that the war against eh Viet Minh was going poorly and they wanted an out. You have to remember that this was in 1954. France was still weakend after WW2, Laos (allied to France) were unable to keep their border secure or push the Viet Minh out, China to the north supplied both expertice, equipment and bases to the Viet Minh and the French knew this (although not exactly to what extent).

So the French knew they were up shit creek and were looking for a way out. Peace talks were under way. France was basically aiming to get out of the war on the best possible terms. Defeating the Viet Minh was not an option but weakening their position, thereby strengthening the French negotiating position was.

So it is perhaps in this context we have to look at Điện Biên Phủ, not as an step towards winning the war but as a way to strengthen the French negotiating position. A few things could do that.

• A strong foothold in the Viet Minh staging area in north western Vietnam would definitely hinder Viet Minh operations in more important areas of the country, namely the Hanoi river delta. Supplies were a problem for the Viet Minh at the time and a french staging ground right in the middle of Viet Minh supply lines was definitely a threat.

• A serious military defeat for the Viet Minh, while not enough to swing the war as a whole, would still weaken their negotiating position. With their options restricted (not enough men for a static defense, not enough infrastructure and too difficult terrain for mobile warfare or rapid response) the French were looking for ways to achieve a victory. The hedgehog strategy had been successful at Nà Sản.

Put very briefly the dream scenario for the French would be to occupy Điện Biên Phủ, cut off Viet Minh supplies from Laos (Điện Biên Phủ controlled one of the relatively few useful crossing over the Nam Yum river), have Giap attack and withdraw with heavy casualties as he had at Nà Sản and then go from there to negotiations of peace where they could basically say "Listen, we have an air base in your rear areas, we've cut you off from Laos so you can't advance further in the Hanoi delta and you just lost 10000 men, let's talk".

Clearly this did not work out as intended for a whole host of reasons.

With this in mind, here are the short answers:

wouldn't the Vietnamese have ignored it and attacked elsewhere?

Well yes but not attacking Điện Biên Phủ would have restricted where they could attack quite significantly and weakend their negotiating position.

If the PAVN had attacked and failed it seems that they would be able to withdraw into the jungle.

Yes they could have, as they did at Nà Sản, however the French were okay with that, in fact they were hoping for it. There was never any realistic hope of Điện Biên Phủ being anything other than a defensive victory and maybe a staging ground for further missions.