I think it would be incredibly risky to blade kill and then transport. Cutting the neck arteries produces a blood spray that can easily reach 30 feet, and until the brain tells the heart to stop beating, or you run out of blood it will keep spraying, and then there will be continued drainage from the brain itself (50-60ml of blood per 100g of brain = ~1liter) There could also be a lot of noise involved, bladder release, maybe a bowel purge too. STINKS! The victim and everything nearby will usually be literally showered in blood, and you will have a pool many feet across which will leave the victim and probably the cutter literally drenched in it. If it was outside, the next day the area will be covered in swarms of flies and ants, and be visible for days.
The problem with transportation is simple. Random Breath Testing. You had an excellent chance of getting pulled over, but if you were covered in blood you had a big problem, unless you took the time to stop for a makeover. Plus, dead blood STINKS. You can smell it, especially a few hours later. Also, WAPOL can pull you over and search your car for no reason. Sorry, I meant a 'license check'. The Birnies drugged, transported, and then killed at the disposal site, and were apparently spoken to by a potential victim while they had another victim 'sleeping' peacefully in the back of their car.
Explaining to Senior Constable Plod that it is your new girlfriend asleep in the back seat after a few too many adult beverages is a lot easier and less memorable for him later than finding a body with its head half off wrapped in plastic in the boot and a driver looking like he just stepped out of Friday the 13th. Killing someone is likely to get our man pretty amped up too, dilated pupils, sweat slime, cortisol stink, shaking hands, jittery voice, fast breathing, lots of 'um' and 'ah' while talking, anxious, rapid jerky movements, walking stiff legged, wants to go (right NOW). Cops are trained to look for that. And he could even be carrying facial wounds from fightback. Getting pulled over with a dead body in the boot would have his adrenaline redlining, and his driving would be crap.
Jayne had VR fibers on her? That means she wasn't wrapped when she picked them up initially. So she wasn't bled at that point. Also, if you transport a cut body that hasn't been bled you then have a large blood soaked tarp and your own clothing that you need to get rid of, and if there was any further leakage you risk a spill inside the car. That is a clean up and disposal problem just as bad as trying to get rid of a body, and a potentially massive DNA sample left in the vehicle. IMHO it would be unwise to cut, and like the Birnies, it would be far safer to drug and transport for a suffocation/strangulation kill at the dump site.
Look at it like this, you get pulled over with a roofied chick in the back of your car. Awkward, but you picked her up outside a pub, she's out of it, and you are just going for a drive to a quiet spot to watch the sunrise until she wakes up and the fun begins. Happens every day. Brand new girlfriend, just met, true love. Good times.
Even if at that point Senior Constable Plod decides something is amiss and he takes the major step of arresting you on suspicion it is your word against hers. What have you actually done that can be proven in a court of law? Essentially nothing. Picked up a chick, but she says you abducted her and drugged her?! If she can even remember what happened from the drugs, the alcohol, and the stress. That's crazy talk. I bet she does this all the time. What was she doing wandering the streets in the wee small hours alone anyway, everyone asks. Hmmm.
You are essentially clean, and claiming to be the REAL victim. You were just doing what anyone would do, and now someone is accusing you?! Of what exactly. Talking any more at this point is simple self incrimination. Zip it. Meanwhile, she has a positive blood alcohol reading, tests positive for a recreational drug, stinks of pub mank, makeup everywhere, has clearly made some very bad decisions, can't really remember much, and is hysterical. Maybe even lashing out, and maybe there was that other time... Mum and Dad have arrived, it's 5.00am, everyone is tired, lots of shouting and crying, rape kit (negative), blood tests, major family dramas. Horrible. Maybe they just want to call it a lesson learned and go home. Most likely, as nothing really happened/we will talk about this at home young lady/how could you be so stupid?!
If at that point the police make the incredibly pointless move to actually charge you with something (that is their option, not the family's (I believe)), the most they can probably hang on you is maybe Deprivation of Liberty (2 year good behavior bond for a first offense). Maybe. If the Magistrate doesn't give the Prosecuting Sergeant a bollocking, and throw the entire case out due to a lack of evidence. Most probable.
The likely scenario is that they will carefull consider that they have an unreliable and possibly uncooperative (ahem) 'victim', minimal to no evidence that an offense was even committed, no witnesses, shift change is coming up - and if there is the usual overtime ban they will then be working for time off in lieu which they will never get and wifey will pitch a fit. As usual. Meanwhile, you have long since chosen to no longer cooperate with them after you elected to invoke your legal right to remain silent awaiting proper legal representation having up until this point declined to utter or sign any form of formal statement.
You might get a stern caution from the East Perth Lockup Duty Sergeant when you leave that morning, with your ribs bruised and your ears still ringing from when you fell over and landed on that phone book three times during informal questioning in the stairwell away from the video cameras. You may even still be soaking wet from when you complained of being hot and they put that fire hose on you in their special holding cell and then kindly turned on that big industrial exhaust fan that they had back then to dry you off a bit. Brrr, East Perth can be chilly first thing in the morning.
And off to breakfast at Valentinos before dropping the car back. Cappuccino or Macciato, that is the question... Easy as.