I just wanted to clarify one thing. There is no such thing called Human/Jinn offspring. Humans are made from Clay. Jinn are made from fire. Jinn are capable of inciting ideas and thoughts that provoke us, human race, to commit sins.
To understand the first verse with regards to sharing, we need to go through the whole verse:
17:64 And incite [to senselessness] whoever you can among them with your voice and assault them with your horses and foot soldiers and become a partner in their wealth and their children and promise them." But Satan does not promise them except delusion.
Your horses: here refer to two kind of categories. Jinn who follow Shaytan "Satin-Iblis" and incite thoughts as he does, or Humans as well who follow his foot steps.
Your foot soldiers: here refer to the same two categories except that they don't ride horses.
become a partner/share in their wealth and their children: partnership or sharing here is not physical in sexual sense. Rather it's actually means you share them by commanding them to use their wealth in something that doesn't get them benefit. Like spending on the Gods as pagan Arabs used to do before islam or using money in Interest which is forbidden by Islam. Or taking money from people by steeling, robbing or etc... Sharing their children in the same form and manner, like pagan arabs used to sacrifice their children to the Gods (Idols they used to worship), burying females once they are born, convincing children to worship the devil rather than God extra. Please refer to my reference.
55:56 In them are women limiting [their] glances, untouched before them by man or jinni.
There is 2 ways of understanding this:
First, we know that Jinn kind are people just like us (Humans). They have the good people and the bad people. The only difference between them and us, they could hear and see us we couldn't. Multiple scholars said that this verse here refers to Men from both kinds (Human/Jinn) who will enter paradise and have spouses from their kind (Human/Jinn). And that's why it's mentioned in that way. The message of Islam came to both Humans and Jinn and the evidence of that, the whole chapter that speaks about them named: Al-Jinn.
Second way of understanding this, it's recommended for husband and wife to say a prayer before coming together to be intimate. Some scholars said if they forgot to say it, they worst of Jinn kind (Shaytan) comes and incite something bad for that fetal that shall be formed in the womb.
I personally, prefer the first understanding.
At the end, that the best we could present and Allah knows best.
References: (unfortunately in arabic)