I guess this is a question for an Orthodontist.

I am doing my second lot of orthodontic treatment using invisalign. I originally had orthodontic treatment, at the age of 18-20 to close large gaps and reduce protuding teeth. Due to not wearing the retainer properly (my fault) and also the age and severity of my initial problem my teeth relapsed slightly.
My dentist at the time explained that given my age/problem it was likely I would always have some degree of spaces in my teeth but I hate them hence the repeat treatment. I am now 29 and had noted that even now they still appear to be moving back slowly (hence why I started repeat orthodontic treatment).

My new Orthodontist explained following my invisalign treatment it would be advisable for me to have a bonded retainer fitted. It was explained that there are few health concerns around this.

However further research by myself has raised further questions:

a) I read that they cannot be fitted to the top teeth, although this is the plan for where they will be bonded as that is where the main relapse is. Is it true they can only be bonded to lower teeth for bite issues?

b)It appears hygiene concerns are more of an issue that my (and other) dentists have lead me to believe?

c) I am aware of the V-Loop retainer which permits flossing. My orthodontist is unaware of this method but says from the description he would have greater concerns about food being pushed up to and trapped by the gum line.
I wanted more info on that because from little I have found on V-loop retainers they seem better for hygiene reasons and I wonder if I can have another Orthodontist fix the bonded retainer.

Sorry for the length of this and thanks in advance for your response!