Advice on finishing first pair of speakers

No on the poly over paint. If you want miror finish w/ paint you have to do wet sand, polish, wax or use a real base coat / clear coat.

For screw head size and larger imperfction filler I like bondo. Cheapest place I found it at was pep boys. For nail heads I use drywall compound.

The sensations being a ported design should be lined with poly, not stuffed. Get quilt batting or an open cell foam matress pad (cheap egg crate type, not memory foam) at walmart and use a stapler or spray glue to line at least the back wall and 4 side walls.

Wet sanding is actually very clean/low dust.

Spackle works for holes. I like a more durable solution for the dreaded edge misalignment which is why I suggest bondo. I've always had good results with it.

The mattress topper only comes it egg crate style at walmart. It's the cheapest and easiest to work with. However, if you find it flat that is fine. The shape is unimportant. 1/2 to 1" thickness is sufficient. I'm not a fan of using anything made of lose fiberglass inside a speaker.

I typically screw down the xover with some rubber between it and the cabinet side so that it is removable. You could glue it in place permanently if you want. Hot glue works well.

My experience from building the overnight sensations (both tm and mtm), you need very little stuffing. The insulation you have is fine. Just spray some adhesive against the back wall and put a full (compressed) handful in there. Wear gloves. You don't need to line all the walls, just the back of the woofer. The amount needed wouldn't cover all the walls anyway. Anything will work though. I personally use stuffing from an old pillow as I don't like insulation.

Always wear a mask.
If you are trying to do this in your kitchen, I recommend sanding edges till smooth and apply a sealant or acrylic primer.
Smooth over with spackling compound (heavy patch) if needed. Apply another coat of sealer.
If you have to use a brush or roller for the painting, moisten the brush or roller first and blot dry.
Highly recommend a textured paint if you've never done this before. Getting smooth finish can be done, but it takes lots of practice and sanding.
I use a block sander, and then sand using my hand around the edges. That way you can feel any imperfections as you are sanding.

Take your time and don't worry. You can always sand mistakes down.

For the crossover just use velcro. The kind you find with the sticky tape on both sides. Cut a small square of the that for each corner of the crossover board.
Place the crossover on the bottom of the speaker.

I use Zinsser Odorless primer-sealer. It can be brushed or rolled on.

I would just practice on a spare piece of mdf. Make sure you get the ends too as this is where its more porous. This is where you will see the problems unless its sealed well.
When you are done you should have the same consistency on all sides and corners.
Then you are ready to paint.

Sometimes I will use drywall compound to fill the edges with my finger. Unlike spackle or conventional wood putties, drywall compound is a little sloppy. But after it dries, it sands off easily.

You can just add sand to the paint if you want that type of texture. There are various kinds you can get too at the paint store.
I find the more texture, the better. It hides imperfections but yet creates a uniform look.

This is just my personal opinion. Do not use glossy paint.