My input here:
1. Where is SPINE located on a NH-29 Series?
SPINE itself is meant to be a wireless connection point between a nekovalkyrja and the backing of a seat or power armor. So, basically, when a NH-29 sits on a chair with the appropriate receptors, information can start being exchanged. Connection is usually done while seated, but has a good enough range for the neko to stand up from her seat. Going further away degrades the link (which is usually cut off then, as a safety measure).
Visually, when I designed the SPINE interface, it was meant to have the dorsal skin right above the spinal cord be darker colored in a birthmark-like fashion in a way which would gradient back to the neko's normal skin color around an inch away from the skin immediately over the spinal column. There weren't supposed to be holes, connection points or anything of the sort.
2. Are tails common among them?
Tails are out of style for a long while now. They were seldom seen on NH-17s. You might consider them completely phased out with the NH-29.
3. From the wiki, there is a compulsory 8 years of military service before being released. Is there other conditions where there is no need for them to be tied down?
The Star Army of Yamatai, in competition to the United Outer Colonies military, tried to improve the conditions for which a Star Army soldier would work under. In an effort to abolish slavery, new nekovalkyrja find themselves no longer under such strictures... though a nekovalkyrja won't go very far from her birthplace if she doesn't have a place to mature, win funds and such. That said, it's still greatly advantageous - and such a good incentive - to have nekos remain in the military becuase that's mostly where they are created and can gain resources to become self-sufficient and the support needed to orient themselves for the rest of their lives.
You might view it as the Star Army of Yamatai being more or less the nekovalkyrja-equivalent of elementary/high school. You earn your basics there for life there. Skip there, and you might be found wanting for everything else in the rest of your life as an individual.
4. During their service, if one happened to be KIA and to be STed, is their 8 year service being renewed?
No, not really. If you get killed and that they end up being nice enough to revive you, it doesn't make the length of your tour of duty any shorter.
5. For NH-29H series, can there be those with human hands, but with cat ears instead?
Not so far. The pinkie is seen as largely vestigial for humans, so, there's little reason to make custom models of that, and custom models beyond the standard base tend to be impractical and costly. I could see a warrant officer with 10 000 Ks on hand managing to get it for herself during a soul transfer procedure... but having five-digits implies a different skeletal structure and that definitely shouldn't come off free.
6. In regards to their hands & feet, can I clarify that their fingers and toes are slightly stubbed, and their feet are more feline engineered as they tend to walk on their balls of their toes?
Aside from the skeletal structure being different because they only have 4 fingers/toes, they ought to be mostly the same. Personally, I don't really view the fingers as stubby - just nailless and as smooth under the digit as it is over it (instead of having a sort of flat groove in which a fingernail would normally grow). They also likely have their feet handled muscularly the same as humans and not felines - I often walk on the balls of my feet, firstly, and secondly I think the habit is more because they are used to dealing with less gravity on starships.
7. Due to the lack of information on NH-12 series as its being phased out. Since they are considered personal assistance, how much?
Fetch papers, fetch pencils, fetch butterknife, spread butter on toast, turn off the oven, drag dirty laundry away, toss refuse in the wastebasket, get into very small spaces to do repairs on things larger people would have trouble getting access to.
That sort of assistance.
8. Is there a timeline for the series, which we can see on the comparison chart?
There was something for the NH-7 awhile ago, but it's likely obsolete by now (and the values used very arbitrary). The
link Yoroko gave you is about as good as it gets.