How do you design the channel work for magma forging to prevent magma crabs and building destroyers from interfering with production?

All magma powered buildings contain impassable squares. If these cover the channeled tiles, it will block entry for the crabs. And prevent dwarfes from falling into magma.

If im not mistaken, the Forge for example has this pattern:

WWW
XWX
WWW

where ´W´ is walkable and ´X´ is blocked. Better check up on the Wiki though. If using this method, there must be no other path "into" the lava, so all channeled spaces need to be covered either by impassable squares or be "floored" from above(I see many such open spaces in your setup). A better way is to channel down a single tile, dig the pipe, let the miner tap the reservoid and get out, then cover the channeled tile with above method.

Also im not sure how this works out for eventual visits from magma-resistant building destroyers.

A failproof option is to separately carve out a section below the workshops and fill the area with a pump that is sourced from a reservoir. The tile where the pump inputs can be covered with a floorgrate which is undestructible from below.

Something like this:

XXXXX
X...XXXX
X...PP#X
X...X..X
XXXXXXX

Where Xis wall, .is floor and P is pump inputting from east. # is floorgrate over channeled tile connecting to natural reservoir. The setup should be safe for manual powering of the pump.

Of course, use magma safe components for pump and floorgrate. That means this requires steel or candy to be available. And it also means that a method of forging the pump components has to be available, which is a bit of a bummer when the original problem is setting up the smelters and forges earlygame. If you add a lever-linked floodgate in front of the pump, you can later close this to restrict the magma flow in order to reclaim the pump components, if steel is a rarity. While considerably more difficult to set up, this method is impenetrable to building destroyers since floor grates cannot be destroyed from below.

XXXXXX
X...XXXXX
X...FPP#X
X...XX.lX
XXXXXXXX

Here F is a floodgate linked to lever l.

Also, as a final reminder, I need to state that any usage of magma reservoirs without the possibility of draining said reservoir is a disaster waiting to happen. Always, before tapping a reservoid, build a floodgate and link it, so you can later interrupt the magma flow.