Can't specifically answer your questions for the 500L as I've not worked on one however I would strongly advise the following:
1) Get a set of trim removal tools. Search on Amazon for "trim removal tool" where you will find plenty. I personally have the modestly priced Winomo set costing less than £10. There are more extensive kits. Usual "right tool for the job" advice.
2) The driver seat heater switch/pressure pad is most likely wired in the same connector underneath the seat that also control the seat belt pre-tensioners, etc. Even if it is not then to remove the seat you will have to disconnect all cable connections SO YOU MUST disconnect the battery and leave the car to stand for 10 to 15 minutes to allow the airbag system ECU, etc. to self discharge. If you fail to do this then you will trigger the airbag systems self checking functions and cause the airbag warning light to come on. This will not reset automatically and you will have to visit the dealer/garage/member here with diagnostic kit to get the light turned off.