Regarding the lifespan of a clutch, the old adage - your mileage may vary - is the key.
Clutches if used properly are like many other components in a car, they have an inverse bell curve for failure rates. That is to say that their greatest frequencies for failure occur at the begining of their useage and at the end of their normal expected lifespan.
It's not unusual to get well over 100,000 - 200,000 miles out of one if it's not abused. I figure I've got over 1,000,000 miles of driving cars (mostly manual transmission) with the highest milegage being around 160,000 and only had 2 go bad. One was a manufacturing fault that showed up within 10K the other was a pressure plate failure after 150k. The stock clutch is more than capable of handling the power of a N/A SVT.
I'm certainly no expert in ths area but your problems with engaging 1st & reverse could be:
- clutch related: slave cylinder and or pressure plate or
- transmission related: syncros
I was wondering, does the condition occur wheither the motor is running or off? If it occurs at both times I'd be more inclined to suspect the syncros but others may be better at diagnosing this.
Regarding possible repairs, I'd lean toward using the dealer IF you have a good relationship with them and feel they've got a very good mechanic (there is no substitute for a good mechanic - low labour rates are usually more expensive if the quality and speed of the work is wanting). Many prefer independent mechanics but if their the same level of expertiese (find the best you can) the factory trained SVT Certified mechanic is going to have the edge on finding the problem quicker and depending on the dealership, being able to stand behind the work better. I have always made it a point to get to know my mechanic and meet with them personally so they have a face and a person to associate with the car.
Because these are FWD cars, clutch/transmission work is time consuming (read expensive) and what I have done and suggest others consider, is to negotiate a flat up-front-fee for the entire job (net of parts). IIFC, the manual has approx., a 9 hour time rate for a clutch replacement so I would look at suggesting to them that you'd be willing to give them the work for 9hrs X Shop Rate X (0.80 or 0.75).
Good luck.
Regards, Alan