Pex Plumbing System w/ Manifold (Entire House)
Pex is typically installed as home run, not trunk and branch. The central manifold gives (mostly) equal pressure to all fixtures, so temperature won't change when water pressure changes like on trunk and branch. Because water pressure variation won't scald or freeze you, home run also allows more latitude with pipe sizing.

Home run pex also gives you the other advantage of pex, freeze protection. No fittings in walls, attics, or crawlspaces means no hidden damage if a freeze occurs, that's probably worth the added $100 in pex tubing.

1/2" pex will have no problem delivering enough water to any standard fixture. Test have shown that pex will perform almost identical to copper in standard household systems at 60' or more: http://www.homeinnovation.com/~/media/Files/Reports/pex_copper_pressure.pdf

You may be able to outrun your 3/4" feed though. If you have 20+ outlets you probably have laundry, showers, and hose bibbs all sharing that 3/4" feed. Turn on a garden hose and do a load of heavy laundry on warm and you'll be approaching maximum flow through that 3/4" line, which I would guess also supplies your water heater, with no pressure for your upstairs shower. That's an extreme case but you should be aware of it. I have done retrofits on 2 houses that I lived in where I ended up with that scenario, but I never experienced the problem because I rarely water the garden and shower at the same time. On my most recent remodel (with 34 total hot and cold lines) I have run a 1" trunk to 2 closets with manifolds, and put the hose bibbs and laundry on their own manifold just in case (using 1"x1"x3/4" tees to feed 6 way manifolds). That may be overkill though.

You can combine lavatory and toilet cold lines. You'll notice the water pressure change but it will only affect your hand washing. It only saves 1 line per bathroom though, so may not be worth it.

Pex should be secured every 32-48", but in retrofits you can't always do that. I have not had any tubes that bang around where I've had to run unsecured pex, but you may. Less slack in the line will give it less opportunity to move too. Long sweeping curves are fine, elbows are to be avoided for flow and freezing concerns unless you absolutely must use them. There are 90 bend supports that hold the pex in a 3" radius if you need tight turns, usually used to make the upturn into a wall. In your soffit you should be able to do sweeping turns except for maybe the upturn into the wall.