See the issue here is that she doesn' tneed any spiderform buffs. She already has healing when her spiderlings attack something, she has automatic magic on hit damage with her own auto attacks, and slightly increased movement speed. If anything at all I wouldn't give her a shield, I'd make her immune to CC for a short duration after becoming spiderform (similar to how spiders can walk across any surface regardless of it's composition because they're interacting with it on a molecular level that goes beyond adhesion). Not a permanent thing, no one should be immune to all CC, but one could possibly make it a constant but have it work only on a smaller number of CC effects, i.e. she couldn't be slowed, snared, or knocked up/back although she could still be stunned, rooted, or feared. This also wouldn't apply to abilities like Ashe's, Anivia's, or Sejuani's as their slows, thematicaly, are from the effect of ice on a body which a spider would still be susceptible to. Failing this, perhaps an armor/MR buff would do her spider form some good, although not one that's quite at the level of Shyvana's buff in Dragon form since that still has to remain unique.
Her human form, though, could use some changes although I don't quite agree with all of it. In particular the spiders. The amount of spiders that you have remaining stay the same when going between her human and spider forms, even though they aren't being used in her human form. If they all do AoE damage or are expended to cause a slow in an area around her, she has to build up all of her spiderlings from scratch, up to 5 at max ult rank, meaning that before she can use her spider form's full damage potential again she's forced to stay in human form long enough to land 5 spells. This means that her spiderform's low cooldown is essentially useless as most players would need to wait for at least 3 or 4 spiders to really deal the kind of damage she needs during the late game. Waiting for all five gates full use of her ult by making her wait for at least two other skills to come off cooldown and hit an enemy. Should any of this skills miss, the wait before her ult is at it's full potential just gets longer. So it effectively goes from a 4 second cooldown to something similar to 8 or 9 seconds depending on how many spiders you want to use, up to possibly 12 seconds or so if you can land everything off the first try. The main focus of her kit is the interplay between her human and spider forms, having to wait for her human form to do anything more than land her basic three spells before using her ult inhibits that flow.
The only change I would make, if anything, to her spiders here is that if you want to use them offensively when she goes back to human form they automatically seek out and make one last attack at the nearest enemy to each of them, scattering like a swarm. They do a small amount of damage but cling to the targets, slowing them for a short time. This is something that would help her teamfight potential, giving her a mutiple target form of CC and some lesser single target damage that can affect multiple people, and incentivize her to stop and think about how many spiders she needs to use in her next spider form. This gives a meaningful choice instead of senselessly gating the full potential of her spider form, as if she waits to have all 5 spiders ready she can have full use of them when she turns spider and back to human if they all survive, or with however many are left. To balance out that she would be potentially waiting longer to use her spider form, let her spawn spiders based on a timer she moves around similar to Teemo's shrooms. Giving birth, so to speak. So every x number of seconds she spawns an extra spider ready for use in spider form next time she enters it, or if she's already in spiderform it spawns immediately.
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