Let’s just say that Invel’s plan to have Gray kill Juvia was flawed from the start. Invel’s expectation that Gray would kill Juvia, turn evil, helping the Spriggan 12 defeat E.N.D. wasn’t a good plan at all. Invel’s wish to protect and defend his emperor was a display of his devotion. The problem with the Alvarez empire is they have a leader who is relatively indecisive about what he wants. Though Zeref is powerful he goes between wanting to live and wanting to die almost every other time we see him. Clearly he suffers but there isn’t an obvious cure for his suffering. No matter how real or unreal Zeref’s suffering is, Invel treats Zeref as someone who is to be served and defended. Invel will protect Zeref even if it runs counter to Zeref’s wishes.
Given the context that Invel is one of the Spriggan 12 and will protect Zeref at all costs it makes sense he would want Gray to defeat E.N.D if possible. But the plan to have Gray turn evil in order to defeat E.N.D only vaguely makes sense. If Invel had explained what Natsu was to Gray and gave him the choice it would have ultimately made for better story and dialogue, then if Gray refused to believe Invel they could have fought or done something else to make it an interesting interaction. Ultimately Invel could never force Gray to fight E.N.D. if Gray hadn’t chosen to do so. However that doesn’t stop Invel from trying to have Gray kill one of his friends and perhaps in the guilt and rage of doing so it would cause Gray to be evil. But there was still a problem with that plan, what if an evil Gray decides to join with E.N.D and destroy Zeref? Just how desperate was Invel? How clear was he thinking? For a calculating man it seems he didn’t really think everything through.
At the very least what Invel’s actions caused were a reveal of Gray’s feelings towards Juvia that have been teased for a long time, as far back as the Grand Magic Games where Erza for a brief moment tells Gray he shouldn’t leave the Juvia question in the air forever. We do get moments of flashbacks where we do see Gray and Juvia bonding, or very much struggling to bond. Though Juvia pushes on Gray to bond he is sometimes cold or numb to the feelings of Juvia. Given his background it makes sense for Gray to be absent, cold and numb. His outlook on life is in stark contrast to Natsu and other members of Fairy Tail. Each Fairy Tail character processes struggles differently. Most Fairy Tail characters have some sort of background or past that causes them to suffer at one point or another. Gray was just always the slowest to move forward or progress at times. Clear evidence of this points to the fact that Juvia was introduced fairly early in the Fairy Tail story and has been obsessive of Gray the whole time. It’s been literally hundreds of episodes, yet Gray just has barely let Juvia know he will think about her more seriously.
At the very least we do get Gray beating the crap out of Invel by the end of the episode. It’s a small token of how Gray truly feels about Juvia.