One premise can heap blame upon self.
The other premise allows us to blame Watch Tower.
This is a false dichotomy, and unhelpful too.
I am now an atheist. I do not believe in any kind of spiritual entities. And I can still blame Watchtower for deceiving me with lies and half-truths, and blame them for taking JWs minds hostage.
With or without 'the spirit' Watchtower can still be blamed for what they do.
And (former) JW can also still wonder what biases and flawed mental processes made them join or stay. Once they recognize the mechanics of that, they are better equipped to not fall for something similar later.
Watch Tower lost the spirit...
Blame Watch Tower, not yourself.
This is an important valid coping mechanism for those of us that have suffered stigma and abuse at the hands Watch Tower.
Is it really helpful for people to continue believing half of what harmed them before?
Those who continue to believe in 'the spirit' and Watchtower having it at some point in time, may fall for the same trap again: they may join yet another (harmful) religion that claims to be the sole possessor of 'the spirit'. Or they may go back to Watchtower if they perceive JW 'have the spirit' back again some time from now.
Either 'the spirit' exists or not. If not, it's not helpful to pretend it does.
If 'the spirit' does exist, some evidence about its whereabouts and activities would be welcome.
You're right though in that some ways to approach (doubting) JW may be more helpful than others. It heavily depends on the person though. Some may be sensistive to contemplating whether or not Watchtower 'has the spirit' as you suggest. Others are more sensitive to other topics, such as evidence that mankind existed long before 4026BC, dishonesty and coverups, (lack of) evidence for the existence of JHWH, evidence for the (lack of) divine inspiration of the Bible, etc.
There is no 'one size fits all' approach.