There is no other way to allow you to threaten with a Whip outside of taking these three feats. The only advantage using a whip instead of a reach weapon will provide is allow you to threaten squares adjacent to you.
While wielding a whip, you threaten the area of your natural reach plus 5 feet
Improved Whip Mastery will allow you to threaten 10 ft. away from you at any time for the cost of 3 feats. If you have any primary spellcasters along, an Enlarge Person or Long Arm spell, will increase your natural reach to reach to 15ft.
According to James Jacob, Using a whip with a natural reach of 15ft will allow you a total attack reach of 45ft. This doesn't affect your threatened range, which will always be Natural Reach + 5ft, for a total of threatened area of 5-20ft.
While it would be very feat-intensive, I would suggest if you want to keep enemies away from your allies, to look at a tripping build.
Any time an opponent moves through a square in your threatened area, you are able to trip them to prevent any further movement. Standing up will actually allow you and any ally in range the chance to beat on them even further with an additional AoO, before they risk being tripped by moving again.
Pick up the Improved Trip and Greater Trip feats to get a +4 bonus to tripping, and let you have a free AoO whenever you successfully trip an enemy as well.
If you wanted to avoid the Feat Tax for using a Whip, while under the effect of Enlarge Person and Long Arm, you may be able to use a Spiked Chain or a Rapier to threaten squares 5ft-15ft with finesse, or any Reach,Trip Weapon will allow you to threaten 15ft-25ft instead.