Eerily calm.
The words that came to mind after reading the first couple of pages. Why was he acting so calm? Why did he focus more on getting to work on time rather than on the fact that he became a bug? After finishing Metamorphosis, I felt like I could relate more to Gregor. His indifference was what caused people to be frustrated but I could see why he acted the way he did. 
Since metamorphosis involves the transformation into adulthood, I felt like Gregor never grew as a character throughout the story. When his dad became unemployed, Gregor just accepts the task of earning money to support his family without a second thought. Similarly, when he becomes a bug, he calmly accepts his new nature and tries to keep working despite certain difficulties. This made me feel like he was spineless. 
Gregor was not the only one who had gone through a metamorphosis. While his was mainly physical, Grete went through a psychological one. At first, I liked Grete because she was the first character to feel sympathetic towards Gregor and tried to help him whenever she could. However, as she neared her coming of age, she started to care more about herself. In the end, I was disappointed and angry about the family’s attitude towards Gregor. The parents focused more on finding a new home and a husband for Grete than on what to do with Gregor, which I felt, was the ultimate betrayal towards Gregor. I was also surprised at the ending because it left me asking, “What, that’s it?” while also pondering on what would have happened had Gregor lived.
I had the same exact reaction at the end of the book. I felt sort of empty like “that’s it?”, so Gregor just dies and everyone moves on. I really tried to understand all the characters and look at the situation through their point of view, so if I did have to choose who I was most sympathetic for it would be Grete (mostly because we are the around the same age and if I were in her situation I would probably act the same way). But like you said she became extremely selfish as the story progressed. When she had her “coming of age”, she no longer thought of Gregor as something that used to be her brother but a nuisance that she was obligated to take care of.
On the point that you made that you felt like Gregor, although physically changed, his way of thinking remained the same. I completely agree. He remained bland and static Gregor except now in a disgusting bug form. It definitely angered me that he was as you said “spineless” and made me think if he did “grow a spine” how would the story have progressed? It would most definitely make the novel more exciting and have more of a purpose.
I definitely agree with you, I was quite dissatisfied with the way the story ended. I thought it was incredibly abrupt. I disliked the way it ended because throughout the story, we never really see Gregor fight for anything, his personality is just one that is “eh.” So his death really reflects upon his character too, he just dies off, solving the family’s problems, still never doing anything for himself. So in a sense, yes I also agree that he was “spineless.”
Although many do dislike Grete because she definitely came off as selfish as the story progressed, I actually commend her for everything that she did for Gregor. I mean, of all the characters in the story, Grete, I have to say, put in the most effort, and at least tried. Honestly though, if I were in her position, I don’t know if I would be able to handle a human sized bug as a brother, so I think we shouldn’t be too hard on Grete.
Overall, I think Gregor lived life practically as a working machine. His only use in life was to provide money for his family, work, work, and continue to make money. After his transformation, he had become utterly useless to the family. It definitely did make me upset that the family saw him as truly a pest later on, because after all that he has done for the family, I really think he deserved some respect.
I agree with you on the fact that Gregor simply took his transformation as his new nature because he has been quite used to his daily grinds without taking a moment to look into himself, but still, I thought his reaction is way too absurd I mean his body has been change and that fact would also affect his work. Considering only work, he is still supposed to take a minute to worry about how he would be able to work with his new giant buggy body.
To answer your question at the end, I think if Gregor continues to live, as a cockroach, everything would still be the same. No communication,no caring, no mercy. Maybe his death ends both his and his family’s plights.Both sides are not able to reach each other and why not just cut off the bond? Gregor’s death ends his suffer.
If Gregor continue to live and change back to human being, his family would be cheerful for comeback of their breadwinner but Gregor would have to go back to his salesman job again which he did not enjoy at all.I believe this outcome will continue his suffer too because his metamorphosis, in some way, forces the family to take over new jobs, which I believe is constructive for the family depite Gregor’s own suffer as a bug.