You're right...space is not a complete vacuum. But, it's very close. The gaseous state is counterbalanced by other pressures...giving the "Space" an almost zero exertion force.
Making it stable and constant...nearly.

Now, the constancy of the speed of light? Well, it is a theory. Given the above explanation is fact.
All non-mass particles...travel through vacuums at "the speed of light." Experiments here on earth...we've bounced lasers off the moon. We've created vacuums here on Earth...no biggy...think light bulbs...

Your own theory of light never being constant...not judging here...just offering academic insight...have you actually tested your own theory? In a lab or otherwise...if not, don't feel bad...millions are needed. Have you drawn up mathematical theorems to disprove the theorems that form the basis of the currently accept theories by the scientific community? How much thought have you actually invested in your analysis and evaluation? Einstein spent nearly his entire life working his idea of light and ENERGY from imaginative thought to theorem...and he worked (thought) right up until his death about actually proving it...it was his theory...he wanted to prove it as fact.

Why is it not fact though we sorta accept it? In the universe of Quantum physics, there are four forces.
Electro-magnetism, gravity, weak force and strong force. Each has been proven individuallly to be mathematically true...but they can not be reconciled as one mathematically. Separately each appears to be true...but, they must be true together or each is called into question...just as you have...That's what Einstein died trying to accomplish.

So, Einstein and innumerable other giants of intellect have been unable to prove their assertions or disprove yours...good for your point...bad for your point too...unless you're a Newton in the hiding...which he actually was...or an Einstein in hiding...which he too was...both though rose to prominence based on their work...have or will you?

Really, no put down...jeez...if only more people thought about these ideas much less even knew about the ideas...what a world we would have

Good job! By the way...I have no idea what all this means either...but, I know I don't know...but I know Einstein harnessed the energy of the atom. And that other...took that knowledge and blew two cities off the earth with atomic weapons. That's fairly good anecdotal evidence for me that he possessed an amazingly intricate mind.

Good luck in your scientific ventures...really.