Stranded CAT5 is easier to handle than solid core CAT5 for alarm sensors. generally there is no problem with distance for alarm cables as long as the cable resistance in tens of ohms
We cannot define what is long
The RS485 standard specifies twisted cable, to prevent noise interference. It does not say how much the performance would be degraded by if nontwisted cable is used
It depends on many factors like distance from interfering signals like mains cables etc so it cannot be guaranteed. If your alarm cable is shielded that does improve the situation, so you wil have to make a judgement based on experience