Your entire argument is "hey, spiders can't do that". It is LITERALLY an attempt to prove a negative. Monolith's entire argument is, "Middle-earth is a vast, mysterious, magical place and we've used Tolkien's preference for not strictly defining things as an opportunity to explore new capabilities for established characters." Why shouldn't the offspring of an ancient evil or immense power have the ability to change shape or project illusions? "But, but, but... Spider!" Isn't a counter argument. It's being pedantic.
You have completely misunderstood the issue with proving a negative. It's not that you can't, and therefore anything goes. It's that you can't, and therefore it's the person with the claim who has the burden of proof.
In this case, you and Monolith claim that the lore supports Shelob being a shapeshifter. As that is not written anywhere in the lore that people have been able to dig up, your claim is unproven, and can be disregarded without any evidence.
Read this for more:
https://en.m.wikipedia.org/wiki/Burden_of_proof
If this wasn't the way we did things, you'd have to assume that anything is true based simply on the fact that you can't disprove it.
Santa's workshop is in the middle of the sun, and its heat actually comes from his factory?
Can't disprove it, have to assume that it's true.
Donald Trump is actually bald, and that thing on his hair is an alien using mind control to turn humanity on itself?
Can't disprove it, have to assume that it's true.
It's actually not an alien, but a wig he was given by Kim Jong-un, which contains a secret radio he uses to communicate with Pyongyang?
Can't disprove it, have to assume that it's true.
With that logic, you can never reach any conclusions about anything. You have to have a certain standard for the evidence it takes for you to accept a claim, not for what it takes before you disregard it.