Hi,
I actually talked about the return URL system of HikaShop with the developer of JDideal 2 weeks ago and he did some modifications in the plugin to make it work like he needed.
We also added a small modification in HikaShop in order to make it work for him.
So first, you might want to download again the install package of HikaShop on our end to get the latest patches if you downloaded it more than a week ago. Also, you'll have to contact the developer of JDideal to check with him where they are with the return system of the plugin.
Normally, the plugins do a JRoute::_ on the return URL in order to SEF it. Maybe the plugin doesn't do that ?
Also, regarding the page "after_end", you can edit it via the menu Display>Views of HikaShop if you want to modify it.