Essay inc! You guys know me too well, it's sweet *tears up*
Both have brutal chaingrab options on you, yes, but I don't wholely agree on it being easier for sheik to grab. While she does have a faster runspeed, ICs don't get around by running. They just wd everywhere, which makes them pretty much as fast as sheik. Also, they can wd into dsmash (sheik only has the semi-scary dash attack from dash) or jab (which leads to grab) as options other than grab.
In addition, given enough room sheik can CG you to like 80% before you reach an edge, but with decent DI she can't actually kill you. She will kill you the next time she gets the opportunity to fair you, but dthrow doesn't lead directly to fair until probably 140+%, which you shouldn't be living to anyways. And with good DI, dthrow->ftilt->fair doesn't actually work at any percentage that will kill you. Given enough room, ICs will kill you from a grab though, as long as both of them are still alive. Dthrow chain to dthrow dsmash/fsmash does kill. With only one IC it doesn't lead to death, but i think it still goes to one-smash-from-death percentage.
Another thing to note that I think gets overlooked is that you have to be super careful about trying to grab ICs. If you don't bthrow immediately once you get the grab, you're going to get dsmashed as soon as you do get the grab. And if you end up grabbing nana, you're going to get dsmashed or grabbed after the throw anyways. Sheik is grabbable. With her ftilt and jabs it's not easy, no, but still more possible than grabbing ICs without dying.
Jolts are meh vs. both characters. Needles > jolts, and 2 icicles, while more avoidable than needles, will still cancel out jolt and one will get through in projectile wars. And they can still punish you for jolting too close anyways.
ICs are easier to edgeguard, I'll give you that. Pika doesn't have tooooo much trouble seperating them either, which works out well, but it's **** hard to approach them and you don't have many defensive measures against them either (no shieldgrabbing, wd->dsmash can go through your ftilt, etc.). Good ICs are super scary for pikachu, so easy to just die from one mistake.