Converting 20g freshwater to FOWLR - please help?

I'm cleaning a 55 next week

a) I'll be using a credit card to scrape off as much old algae as possible, and a hose in the back garden. I'll likely use a little of bleach, then lots of tap water.
b)No idea. I'll put in oolitic aragonite that's dead and let bacteria from the live rock move in.
c)Depends on where your live rock is coming from. I will probably get mine partially from an old tank, and partially from a local lfs, where it will be cured , so putting it in to a stable tank will 'instacycle' it.
If you're buying live sand, and putting live rock in that's cured you shouldn't need to cycle. If your liverock is going to turn up in a horrible state, the process of curing is effectively cycling.
Don't get caught up thinking about a freshwater cycle for saltwater. You're buying your filter bacteria in on the live rock and sand. You need to think about what to do to ensure maximum survival of other life forms on the live rock.
d) Yes. More circulation the better, and skiming too if you have it. Get too pumps pointing at each other or crossing in the centre of the tank for turbulence.

Good luck , have fun, go slowly. If you wait 2 months plus to add fish your life will beso much easier as the tank will have stabilised by then.

Uncured often is cheaper--but it will mean you want to do water changes during the cycle, so the ammonia spike won't kill off much of the hitch hikers you get. Works fine, and will result in a good bacteria colony.

You don't have to buy live sand at all--and if you're talking about the pre-bagged stuff that sits on the shelf, I'm pretty cynical about the 'live' part--you'd be better off using all play sand and letting the live rock colonize the sand (which does work, I've done it several times--better live rock = better sand).

2 MJ400's should be fine. I have one AC200 in my 10, and combined with the return flow from my sump, is almost too much.

In small, lightly stocked systems, more frequent water changes can replace a skimmer. I do about a 2 gallon change on my setup (about 20 gallons total water), and haven't had any water quality problems as all.

For cleaning--soaking with hot water and oxyclean is my all time favorite method. Rinses clean, no bleach smell, breaks down algae (even the hardest stuff).

Cured or uncured--won't matter, you'll want to let it settle a bit before adding live stock with cured rock, and with uncured, well, I'm in favor of the increased life you get.

You can get everything in there now--but keep in mind that moving water drops particulate faster than still water (think slow river vrs fast rivers--which are clearer?). So, while some settling will occur without the powerheads, it won't be much, and you'll kick up some stuff when you add them, anyway. Patience is a virtue in other hobbies, but a requisite for success in this one, so don't worry about the delay.

Try to buy all, or at least a lot of rock at the start. You can always add more, but the effect of adding more than 1 or 2 pounds at a time to a small tank like a 20 will be pretty catastrophic unless you can get it totally cured within say a 20 minute drive from your home. Otherwise dieoff will pollute your tank.
When all this is done and stabilised at ammonia zero et al, you can theoretically add fish. However I woulw wait some more, because there's still a lot going on. In the first 6 weeks you should expect to see diatoms (brown algae) appear then disappear, then maybe bits of green and brown before purple coralline starts to appear. These algae problems tend to be trivial if you don't have fish in the tank. If you wait till ammonia is at zero in week 2 or 3 or whatever, and then stick in a bunch of damsels I can guarantee these problems will be much bigger.
Newbies crossing from freshwater always have hangups about, and only think about the nitrogen cycle. This is also important in salt, but it's not the only cycle going on.
So I would wait 6 to 8 weeks while the tank sorts itself out. If you want something to look at get some blue leg hermits, or a cleaner shrimp.

I'm still gonna use the 20g tank, but I'm gonna try a sump (10g big enough?) and skimmer

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Great idea with the sump. If I were you, I'd get the biggest sump you can possibly fit, even if it means custom-bulding one in an odd shape to fit wherever. a 10 gallon tank with a 100gal sump would be great, because you get the great stable temps and other water parameters, and you save a lot of money. It's a lot cheaper to put 50-80 watts on a 10 gallon than 500-800 watts on a 100 gallon. etc, etc, etc. That way, you can jam-pack your 20gal display tank with life, tons of corals, inverts, maybe 1 more fish than usual( provided they'll all be happy and get along in the limited physical space) and your water parameters will be rock solid. 10 gallons IS big enough, but I'd go as big as you possibly can.

[EDIT] Oops, I forgot, sorry, I don't know anything about skimmers.