Wow those are a bunch of questions.
Sarah say's to needle the scar and a little around it so I would not leave parts of the scar unneedled.
Needling alone is the main thing that will cause the magic to happen. The things you use prior (vit. C, A) will help the process. You don't want to use C or retin-a immediately after single needling. Use those in the days prior. Infadolan is perfect to use after. I suggest more reading here about Infadolan. Retin-a is awesome if used correctly. Many don't and complain that it causes too much irritation. Start out with the lowest strength and use a couple times a week and you should be fine. Gradually work up to .05% strength every other day. Infadolan can be used as often as you need it after needling. But it is greasy so most only use it for a short while after. Apply the vitamin C serum 4 days prior if you just begun, or many days prior, or dont use it. The magic is in the needling.
You could use a toner to rid your skin of oils. The vitamin C is not washed off the next morning. It will be absorbed into your skin and cant be washed off. The benefits will last up to 72 hours. This is all assuming your using a quality C serum.
Oral supplements help with general health but to boost collagen production and scar remodeling, your not going to see much, if any, effect from taking supplements. They are working, but in the long term.
I would only be concerned about touching, rubbing the area just needled in the immediate hour or so after needling.
Yes you can apply vitamin C and Infadolan to hypertrophic scars.