1) CPW if fine. Please ensure that the trace from ANT pin to the Antenna feed point is 50 ohms.
2) The Ltip values mentioned in AN91445 assumes FR4 PCB. So, far a different substrate, the dimension could be different. Continuing Meandering is fine, but as the optimal length depends on the thickness and substrate material also, it is recommended to get it simulated.
3)The trace that runs parallel to the meander is a short circuited stub to compensate for the parasitic capacitance formed between the ground and the arm of the antenna that runs parallel to the ground. The low thickness does not affect the performance if the length of the Antenna is suitably adjusted.
Is there any free software you recommend, that could be used to simulate PCB antenna with possibility to add TOP/BOTTOM layers of substrate like FR4 or polyimide or custom defined, assuming, we know the permeability of the material ?
Our antenna will be incorporated inside membrane keypad.