Here some thoughts on what you’re asking. Sorry I don’t have definite answers…but just somethings to consider.
1. Before spending money on a new SCA (soft close actuator)…just this past week…I saw a video (on youtube) that was brought up on another forum about their soft close actuator…the video showed how a non-working SCA was fixed by replacing the screws that secured it to the door. When they removed the door panel… 3-4 screws that held it in place had rubber washers that had degraded….and the SCA could be moved back and forth. When they replaced the screws, the SCA started working again. Unfortunately the video doesn’t actually say WHY the SCA stops working. It was something I put into my mental Rolodex to check before replacing the SCA. So just sharing that tip…it may or may not work…but replacing the 3-4 screws is a lot cheaper than replacing the SCA. So check to see if your SCA is loose.
EDIT: Just went to YouTube and typed BMW soft close replacement to see if I could find the video referenced above…it came up as the 6th result on the list of videos. It was a 7 series that was being worked on but the basic design architecture is similar for the 6 series…so again, check to see if the SCA is loose and has similar securing hardware…you may be able to save some money and easier fix if you find a similar fault:
2. CA (comfort access) door handles is a common repair fix across multiple BMW platforms. The handles are NOT coded…and yes, they can drain the battery. A simple google search for replace BMW comfort access door handles should give you some ideas/suggestions etc on how to proceed.
3. Yes, again, CA door handles can drain a battery. How bad can it get? Any deep discharge of the battery lessens its life span. Do you have a trickle charger? If yes, then keep it hooked up to the dedicated jumpy start terminals in the engine bay to prevent the battery from going into a discharged state. Again, repeated deep discharges damages the battery and shortens its life span. I good trickle charger/battery maintainer (this is also a much discussed topic that a simple search will give you some good advice on what to look for etc) can help you keep your battery in working condition and lengthen its usable life. Or you can take your chances and have a battery that might leave you stranded if left in a situation that it can’t handle (extreme heat or cold spell).