In my opinion, Suggestion (and charm person and the like) are all about one thing
Spin control.
It's not what you ask/tell someone to do - it's how you spin it. The phrasing, the motivation, that's where the save modifier should come from.
With Suggestion/Charm, you have someone who is, by definition, open to input from you, and has a certain suspension of disbelief.
The party breaks into a basement under the flour mill where the bad guys are hanging out in the Return to the Temple of Elemental Evil. Axes and blood start flying as the melee ensues. The bard charms one of the bad guys.
a) The bard's player says "I tell him to attack <that guy, his former buddy>"
b) The bard says "My god they've been tained by the dark evil under the temple. Don't let them get close or they'll try to sacrifice you too!"
The result may not be precisely the same, but the second example gives the NPC a reason, and feeds off things he knows. A DM could assign a save bonus for a) but decide that b) was good.
Or what about
c) The bard says "Quick! Come here behind this toppled table. I don't want you to get hurt in this mess!"
The NPC would surely do this - it feeds on his survival instincts, and follows the line of treating the bard as a friend.
Encourage the players to spin the situation, and reward them for doing so.
To your example:
Well the situation that happened was that a sorceress cast Mass Suggestion on us and said that "Your party is polymorphed drow elves waiting to stab you in the back at the right moment. Kill them." I was trying to state to the DM that it wasn't very believable, though we had encountered many polymorphed creatures the party had pretty much stayed in tact and that as we were sitting there defending ourselves from an attack of Drows that it didn't make much sense.
When you say "you had encountered many polymorphed creatures" does that mean in your lifetimes, or in that particular adventure?
If you had been working your way through caverns of drow who were frequently disguised/polymorphed, then it's very believable. If you just mean you'd seen polymorphing in action before, then maybe it's not.
If you had a group of drow hounding you for the past 5 levels, then it's believable. If you never saw a drow, then it's less believable.
Break Enchantment and Dispel Magic have caps to what they can break.
Example: 20th level wizard casts Suggestion. The DC to break this is 31. Yet with Break Enchantment the max you can get is 35, and dispel magic won't work at all, and this is just for a 3rd level spell.
That's what Greater Dispelling is for. For a 20th level caster you have to figure you may need something more than a 3rd level spell to counter him. Also if the caster is 20th level, we should assume the PC's are also near that. Those PC's will have a fairly easy time making their save against a 3rd level spell, and that's the counterbalance.
What about protection from evil? Prevents mental control. First level spell. There are other ways of course