[Saki] seems to think She's now only using Kanon for her own personal gain to get into a relationship with her which is very silly as that was NOT the reason she started hanging out with/helping her in the first place that just happened over time.
I disagree with this tbh. I think Saki started crushing on Kanon from the moment they met, and that's why she put so much effort into befriending her. (Well that and she was lonely and heartbroken.) If Kanon weren't as pretty as she is, it's unclear whether Saki would've put in as much effort as she did. I don't say this as a criticism of the character, it's normal to try harder to get close to people you have a crush on... that's just kind of how it is.
I don't think "taking advantage" is the right way to describe befriending someone because you're romantically interested in them; it's more the concern that a friendship based on false (or unstated) pretenses is inherently unstable. This is a particular concern here because Saki is basically Kanon's only friend; losing her would be a really big blow, and there's a risk of that happening if Saki confesses and gets rejected (or vice versa from Kanon's perspective).
Just befriending, no. Befriending someone with a disability with that sort of ulterior motive CAN be seen as having potential for exploitation.
That's fair. We've seen that, especially when they go outside Kanon's house, Kanon is a lot more dependent on Saki than the other way around. If Saki were a bad person, she might be able to coerce Kanon into a relationship by using the implicit threat of leaving her if Kanon refuses.