I don't know if you've ever read the comic series Saga but I'm getting a lot of Prince Robot vibes, mainly from personality and appearance. If you haven't then you should definitely have a peek.
I have not, but it certainly sounds interesting!
For integrated chassis, is Spark also effectively unable to age?
Within the universe Spark is in, Spark is capable of regenerating its chassis at will, and recall its personality to its home. Here, however, Spark doesn't have this capability. While the chassis might not rust, if Spark sustains too much damage over time, even with its repairing capability from the welding tool, parts of the chassis may break permanently pending Spark finding an alternate solution, although this would be a bit of a ways off into the future (depending on damages, of course). As for Spark, the personality itself will effectively not age for the most part. This doesn't really mean much unless Spark finds an entirely new chassis, however.

Does his head count as a limb to be 'popped' off?
Yep, although this leaves Spark completely blind and disoriented.

For Plasma power cell, am I right in reading through it that it is basically mainly a weakness?
Yes and no. While it sustains Spark and provides a constant self recharge, if it doesn't discharge the excess, there are severe consequences.

Are there other ways for Spark to discharge other than his electricitt control abilities?
The only other method right now is the wired power adapter (which is broken presently, and would also require a port to insert the cable into).

For electricity control itself are you able to describe the amount of voltage each of the attacks are. You could do this in the form of a real world example (e.g. static shock) or a damage roll dice if you need (e.g. a d6).
I'll dig into Spark's original universe a bit more and see if I can give you a more precise answer on this a tad bit later if that's alright, that is a fairly interesting question I hadn't considered, admittedly :P

Spark 5.5 seems perfectly fine for a non combat companion for now.
Sweet!

Overall, Spark is meant to start out fairly weak. Over time, it will have to adapt utilizing Althanas' resources (magic, for example).
Thanks for looking at my profile!