There is a small "feather" key the rolls along - skims - the 1st gear that lifts out and prevents the 1 gear sync from letting the two gears mesh unless it is below a given speed...
The problem is - how is that speed determined?
Simply put, the gears churn oil as they work, as long as 1st gears are moving they pull oil up and over themselves - so the feather key is a simple lever that is lifted up and prevents the synchronizer from letting 1st fall into place - 2nd gear is not affected by the feather key position. The key is lifted by the gear oil pressure and simply blocks the 1st gear side from engaging - but does not keep the synchronizer from operating 2nd gear.
The spin of the gears turns the key but the pressure of the oil lifts the key up out if it's keyway - the missing piece of the puzzle, when the gear lube skims over the gears teeth - throwing the lube into the keyway keeps that key from letting the synchronizer to fully seat into the gear set - but lets the synchronizer press the gears into a mesh pattern but is locked out because the key is lifted and turned towards the groove the synchronizer would use to finish the throw action onto !st gear - due to the dynamics-of the required torque and spin rate - the feathered key and it's design keeps the meshing from completion until the pressure or spin has dropped and the key can then set into keyway - freeing up the groove to let the mesh drop into place
You can experience that during the downshift, you should be able to hear the whine from the over-spin as the synchronizers pull/push the gears to meet - happens in every gear. However, as you may try; if you try to mesh 1st - remember the output pinion and ring gear ratios - they affect and become multiplier of the spin rate - making matter worse is they went with a 4.12 to 1 Pinion to Ring gear ratio - versus a lower ratio of 3.8x to 1 to help reduce the multiplied spin that the pinion is not making 1st Gears try to spin up to to meet - it's not just the engine side, it's the drive-shaft that adds the spin to this mess.
The gears are massive compared to something like Toyota - where they remove some of the useless mass in the gears to help them spin up and down faster with to less inertia and momentum working against their synchronizer.
Not sure if you had time to verify oil levels - for the IB5 is kind of fussy in that - both in viscosity - thinner oil when its warmed up lets the key fall faster - and less oil churn (foam) that helps keeps the key submerged in fluid not made buoyant from foam or thicker fluid volume amidst pressure from excessive oil levels in the case; making some simple changes can offer some help in letting the 1st gear engage faster.
You're talking to a guy that got stranded from a small plastic clip that broke in the cable box at the transmission - a yellow clip came off the Gear Selector Fork cable - making it impossible to shift the car in or out of gear while driving - immediately puts me on the side of the road and unable to figure out how to get it towed without damaging it further let alone what the hell went wrong with it.
Had to have the transmission replaced due to the amount of damage that happened - the above is my older transmission gearbox selector lever cover showing the two gimbals - before the clip broke.
What does the clip do? Keeps a set of serrated teeth on the Fork selector cable - your side to side motion - to help the forks line up to engage the gear you push or pull into - engaged on the lower gimbals. When the clip is gone, that part of the shifter forks alignment is shot - you're at the mercy of the gearboxes own floating selector rod and its forks shifting into whatever they want.
So, I'm no expert but the reason why I wanted to post in here was to help you find options that may help you enjoy your transmission better without having to have it torn apart and figure out what you really wanted - was a new start and work with that.
Basically, the Fill Plug shown in the photo - is really a level check - if fluid drains out - you've got either too much fluid or you have oil churning which increases the volume of the fluid and both affect how the 1st gear syncro and key will work together,
I've done up a copper wire from an old House wiring cable - using the white wire, and use a convenient length to reach the fill plug and made a L-shape of 1cm or about 1/2" inch on the end so when you stick it in; with the L portion on there down - you can use the L portion as a dipstick to tell you how much fluid is really in there. If it touches the lower tip - you're all set but if it comes up some distance of the L you may need to use a turkey baster and some vacuum tubing as a means to suck out extra fluid to help with reducing the foam, or changing / displacing some fluid to add in modifiers so you can change how the fluid acts in the gearbox.
Another thing too, so we are on the same page, run your VIN. Because if they had to change the Gearbox to meet specific sensory needs like speed sensor or Traction Control - they did make up a Limited Slip Differential gearbox option for Fiestas.' - so if you find out on your VIN you have a LSD - it may explain the reason for the difficulty in having to downshift - for LSD differentials and traction control on icy pavements when you're trying to downshift into first - can damage the LSD from it trying to take up slack from the syncros trying to spin down and the resulting torque hammering on the drive-train due to it.