OK, I'm not going to make too much of an argument just point out a few flaws:
1. Chemical warfare is allowed but seeing as Revan has shown no possession of chemical weapons it is not apart of his power base and therefore cannot be used.
2. No, the MSG is not active and cannot be activated. I think you misintpreted when I spoke of this, when I mean you can't turn it 'off' I meant its effects can't be stopped. Even after its activated its the mass shadows continue to remain around Malachor causing its instable gravity - so ships could only stay there for a small amount of time.
3. Orbital bombardment is possible, but not if you don't no what to bombard. The Trayus Academy is a tiny speck on Malachor's surface and Revan does not know its exact location. I'm going to make an assumption here but I expect electrical storms would prevent Revan's scanning the planet for the Academy. And while he could scour the planet with the Force, both the dark side and Traya herself would cloud him vision. Sending ground forces is the only option.
4. Defections are unlikely. A majority of Traya's forces are mindless sith who have been become enslaved to the dark side. Whats more Malachor doesn't exactly have holoprojectors, how is Revan going to trasmit this message? However many Sith are more autonomous and while they would never join Revan, they would flee the battle (however they have no where to go - so a fight to the death is likely)
5. Just to address a non-Battle of Malachor related topic. People seem to think the Jedi's devotion to Revan will stop them from falling. Wrong. Traya's isn't going to bribe you, she's going to torture and force you into submission with the overwhelming power of the dark side - as Revan did in the JCW and she did during the First Jedi Purge - its tried and proven. (e.g. Remember Bastila? She was turned without Malachor, and had no motives for joining Malak)
6. Revan cannot throw debris at Malachor, lol. This isn't that 2D Clone Wars series.
And on the topic of unorthodox orbital bombardment - the gravity of Malachor would prevent ships from crashing into the academy, they would have no control over their path. May I also add that the Trayus Academy is submerged into Malachor's crust. It survived the Mass Shadow Generator! And thousands
of ships being smashed against the planets surface! It can survive a brief orbital bombardment.
Basically orbitally bombarding the most inorbitally bombardable planet in the galaxy is not a sound tactic. Open ground warfare however is
However there are three things I'd like you to consider on that point: