As for Scarabs, I can see its main gun being able to destroy a Leman Russ, but let's be honest: The projectile is slow-moving and easy to avoid, with a dissapointing lack of AOE to compensate. Guard and Tank commanders will eventually catch on to this, and learn to fire their tanks on the move. A Warhound Titan is redundant, all that's needed is concentrated Battle Cannon fire, Demolisher Cannon fire from a Baneblade, or a Basilisk/Manticore bombardment.
And I think you're forgetting the sheer over-the-top-edness of 40k. The Imperial Guard is NOT the UNSC forces. Each Guardsman is zealously devoted to the omnicide of all non-human sentient races, and equipped with weapons that put half of the UNSC arsenal to shame...and that's just standard issue. I have no doubt a single guardsmen could gun down an Elite without much difficulty (though the energy uzi might do some hurtin' too)
You are right in the OP when you said it's a blowout, but i'm not sure you truly appreciate how massive a blowout it would truly be. With the scenario provided, it's pretty easy how the battle goes. The IG holds the planet without much difficulty, the Marines send a single company, possibly a Terminator Boarding Squad in to destroy High Charity from within, and they succeed with the Covenant being powerless to stop them, and their fleet is decimated by a huge Imperial armada of ships that are built to deal with threats of far greater magnitude than the Covenant could ever hope to bring.