I've never used auto-clutch in the 12 or so years of playing ISImotor sims. I sometimes lift on upshifts (depends) and of course blip on downshifts and regardless of whether I'm using sequential paddles, sequential stick, or H-pattern.
Of course I don't have to lift and/or blip for all cars as many cars have auto-lift and/or auto-blip set in the physics.
The only exception to all this is when a car is supposed to have auto-blip and/or auto-lift but hasn't been included in the car's physics. In this case, the "generic" auto-blip and/or auto-lift settings have to be used. The only way to use them is by enabling auto-clutch and then making sure auto-blip and/or auto-lift is enabled in your player file.
BUT, with regards to my very previous paragraph, to make things even more confusing, I think some cars without auto-blip and/or auto-lift can be set to ignore your auto-lift and/or auto-blip settings even if you've enabled auto-clutch. The Historic F3 is an example of this if I remember correctly; you can enable auto-clutch and have auto-lift and/or auto-blip enabled in your player file but the car will still not use them.
Basically the whole system is a convoluted mess.
What if you haven't got a clutch pedal?
Don't need it. You only need it for when you're stopped. So just map a button for clutch. Then go to the digital sensitivity settings for your steering/clutch/throttle/brake - not the one where you adjust the steering and pedals sensitivity and deadzone but the one where you adjust the "digital" sensitivities of each - and play with it so as to have the clutch release at the speed you want it to when you release the clutch button for the start.
I usually use 20% to help with slip and not bog but some cars/gearing may better suit a lower digital sensitivity like 10 or 15 percent which will make the clutch fully release quicker when you let go of the clutch button. You can visually see the speed of the clutch button press on the clutch meter in the controls section. Play around with it.