You have been on the Camino several times. What is so different about this time?
The advice from the forum is "all over the map" so you will just have to pick something that seems to suit you best! There is no clear answer, and it seems that most people do not get bitten by bedbugs. For those who do (like me) it is usually just a few bites. If we weren't so paranoid about the
idea and the yuck factor, it would not be such a big issue. The good news is that bedbugs are easy to kill - heat them up to about 50C or 120F for 20 minutes.
Furthermore, bedbugs do not bite through fabric, whether silk or cotton, whether sleeping bag or liner. They find exposed skin. That is why I suggest insect repellent on your face, neck, hands and arms, if you are concerned. Rather than a spray, I prefer to take a lotion that can be applied with more care.
The decision to take liner or sleeping bag should be based on which will be more comfortable for you to sleep in. You are the best person to make a guess on that. I don't spray my bag with permethrin, but you might choose to do so. I now carry a very light fitted sheet that is sprayed, and use my sleeping bag on top of that. Permethrin is presumably quite safe, but I understand why many people don't want to use it
Get a large dry bag (
for example like this) that your whole pack will fit in, at night, to keep bugs from entering your pack when they are more active at night. Or use a plastic garbage bag, tied at the top. Get another dry bag (or plastic bag that can be tied tightly) to put your sleeping stuff into during the day. If no bites appear by the time you reach your destination the next day, you are probably safe. If bites appear, then you should go straight to a clothes dryer and put all of your sleeping stuff (put it dry, without washing, for best effect) for a cycle.
Finally, decontaminate everything before going home (or carefully immediately upon arrival). See
these comprehensive guidelines.