tiawd:
Your reckless claim that an explicit statement of the Baal Hatanya is “Kefira” is the main problem with your post.
The Tanya specifically adds the word “Mamash”!
To say someone who says Mamash does not mean it literally is simply absurd, dishonest and False.
Your first “proof” is actually the biggest proof to the contrary of your claim as the real meaning of אין עוד מלבדו as explained by the sources you quoted is that there is no real existence outside of Hashem.
Take a look at Sforno Bereishis 32:1 who also alludes to this idea.
The distance you are trying to create between yourself and Hashem is actually a lot closer to the “border” of Kefira (Christianity) than the idea you are questioning.