I have talked to quite a few people about their experiences with visualising things, not just seeing, but also hearing, smelling and so on.
Of course there was some external input at some point to give your brain the memory of something, for example a favourite cake of yours, your mom used to make when you were little.
Most people I talked with would be able to visualise that cake, seeing it more or less vague, but also getting a sense of smell and sometimes taste too. For some it would be pretty much like having taken a bite of the cake a while ago and still having the lingering aftertaste without the cake actually being in their mouth. I get nothing like that, not even vague, just nothing.
Quick going over your points:
* 'what is the step-by-step process of sense-perception?'
One or more of your senses get/s input/s from outside, be it light, soundwaves, smell particles, sense of touch etc, and then processes this to make you see, hear, feel, taste, etc things.
* 'Is there really no external input? Are other senses feeding into the visual sense at all?'
As I said, of course there has to be initial input at some point, you can not get the sense of tasting a strawberry if you never tasted one. But someone who can synthesise sense perception in their mind could probably go into an isolation chamber and easily visualise strawberries and what they taste like.
* 'what is the person's real conception of what they are doing?' (as opposed to the usually very vague words they use to describe it)
I think this is the biggest issue. I have read about people who have aphantasia never really thinking there was such a thing as "the mind's eye", it just being a metaphor. Then finding out that, no, it is not just a metaphor and there are people who can manifest visuals for themselves with their minds is pretty crazy sounding.
* 'In what ways are two individuals' sense perception comparable, and in what ways are they incomparable?'
I would say that they are comparable in that you can hook people up to an EEG machine and see what their brain is doing while they do things like looking at stuff, and then things that would lead people to visualise things, like reading for example. Of course you could always make the argument that you do not know what other people perceive, and my green might be totally different than your green. I however think that from what we know about evolution and neuroscience among other things, we can be fairly certain that most people, those who fall into the "norm" spectrum, perceive things very similar at a sense level. How they interpret things of course will vary due to factors such as upbringing, culture, biases, what have you.