It is well known that each $n\in\mathbb N=\{0,1,2,\ldots\}$ can be written as $2w^2+x^2+y^2+z^2$ with $w,x,y,z\in\mathbb N$. Motivated by this, here I pose the following novel question.

Question 1. Can each $n\in\mathbb N$ be written as $2w^2+x^2+y^2+z^2+xyz$ with $x,y,z\in\mathbb N$?

I guess that the answer is positive, which has been verified for $n\le 10^6$.

Similarly, I have the following four questions.

Question 2. Are $7$ and $487$ the only natural numbers which cannot be written as $w^2+x^2+y^2+z^2+xyz$ with $w,x,y,z\in\mathbb N$?

Question 3. Is it true that each $n\in\mathbb N$ with $n\not\equiv 3\pmod4$ can be written as $4w^2+x^2+y^2+z^2+xyz$ with $w,x,y,z\in\mathbb N$?

I also believe that the answers to Questions 2 and 3 should be positive, which have been verified for $n\le 10^6$. It is easy to see that $$x^2+y^2+z^2+xyz\not\equiv3\pmod4$$ for any $x,y,z\in\mathbb Z$.

Question 4. Is $23$ the only natural number which cannot be written as $w^2+x^2+y^2+z^2+3xyz$ with $w,x,y,z\in\mathbb N$?

I guess that the answer is positive. I have checked this for natural numbers up to $2\times10^6$.

Question 5. Is it true that each $n\in\mathbb N$ with $n\not\equiv3\pmod4$ can be written as $4w^3+x^2+y^2+z^2+xyz$ with $w,x,y,z\in\mathbb N$? Are $7,\,87$ and $267$ the only natural numbers which cannot be written as $w^3+x^2+y^2+z^2+xyz$ with $w,x,y,z\in\mathbb N$?

It seems that Question 5 should also have a positive answer; I have checked this for natural numbers up to $10^5$.

Any ideas to the above new questions? Your comments are welcome!