Porcini is a fairly strong flavor. I can't imagine much of a difference in simmering 3o minutes or an hour. I would start with whole, good quality (there is some variation) dehydrated porcini. Soak or simmer. Then, I would squeeze out the rehydrated porcini. Strain the soaking water through a coffee filter to remove grit. Put porcini and filtered soaking water into a high speed blender. Blend. Then strain and add to your pate. Just know that porcini will be the dominant flavor of the end product, but maybe that is what you are going for.
I would switch to making it in a pressure cooker. That way, you definitely get much more taste out of it than by simply simmering.
You cannot add anything to "help draw out the flavor", but of course you can add ingredients which contribute their own flavor, such as salt, marmite, etc. It is better to add these after the broth has been made. I wouldn't add oil to the broth; add it to the final dish instead, at the appropriate time.
You can use a dashi infusing method with konbu (dried kelp), unheated or heated, and check the taste each step:
Try a 5:1 ratio by weight porcini powder to konbu to start, in 20 parts cold water. The mushroom powder may have lost some flavour due to age with a higher surface area, so experiment with a higher ratio of mushroom powder.
Steep the powder and konbu, preferably overnight and in the fridge. The long cold steeping will extract the flavour without extracting alginates in the konbu that would affect texture. Do a taste test here.
If desired, bring the mix to a very low simmer and turn off the burner for heated extraction. Allow the mix to cool and settle before decanting.
Konbu with its glutamic acid has a synergistic effect on taste with the guanylates from mushrooms. Other foods you can consider mixing can be found on umamiinfo.
Adapted from a shiitake mushroom dashi recipe from toiro kitchen.