I've made a small werebeast experiment, with an save archived from the moment a wereelephant attacked.
5 dwarves were bitten and survived. I isolated them. 4 of these were infected and changed. The one uninfected dwarf was bitten in this way:
The wereelephant bites the performer in the left hand and the severed part sails off in an arc!
So biting off a limb doesn't guarantee infection it seems (which was unfortunate here, all other had some of their limbs gone... for a month).
The wereelphants dropped their crutches, armour, clothes and weapons+shields (two had a weapon: one had a crossbow, the other had a battle axe, both had shields, the weapons and shields were the first items re-equipped later, despite not being at the top/bottom of the heap). The wereelephants had normal arms, hands, legs, feet, tail and trunk. But they did not held their weapons. So having an arm+hand doesn't quarantee keeping weapons. Maybe they were too small?
Sidenote: one of the infected was a mother with baby, which was unharmed in initial attack. I run three tests. In first one, the mother killed the baby after transformation, and then destroyed the door she was locked in (of course she was also walled in, so she couldn't do much more). During second and third test, the baby wasn't attacked at all, and was carried by the wereelephant. The mother destroyed the door, waited for the transformation back, and carried on as normal, with baby untouched. So having a were-mother doesn't guarantee killing a child, which is a nice touch. Or a bug.
I think that were-beast are to be similar to humans, or animal-humans, as the lore goes, so I'd expect them to have normal limbs. After all, they are "twisted into humanoid form".
Additionally I checked with dfhack, and the only body parts the wereelphants had with "grasp" flag were their hands. Apparently they cannot use trunk for this. I also noticed, that wereape, weregila, werecoati, werelizard, wereiguana and werechameleon have "FINGERS" (and toes) in body plan in RAWs. Other werebeasts from my world do not have fingers. Maybe fingers are needed for keeping weapons as werebeasts? Fingers of dwarves do not have "grasp" flag too, but maybe there is code for this. Can a dwarf keep weapon without fingers?