As far as I understand, in addition to the main targets, Israel also knocked out a good number of the air defense systems Iran had at the time.
Given russia can't even replace their own losses of sam systems I personally doubt that they are sending much to Iran.
It is an inherent property of low frequency radar that it is not precise enough for weapons targeting. You can't engineer that away.
That being said, these aircraft aren't invincible. If you could visually see the B2, you might be able to essentially hip-fire a SAM at it and hope it eventually picks up some signal as it gets close, but that would be pretty unlikely to work and some missiles might not be capable of doing that at all. There are some soviet systems that are optically guided, and those would probably work fine. In the day at least. Heat seeking A2A missiles would probably also work, though the B2 has some mitigations there. All these options are a significantly limited engagement range though, and I would hope B2s aren't sent alone. A few F35s with the strike would fix all problems. SEAD is way easier when the enemy's radars can barely detect you at 60km. US anti-radiation weapons can reach out to over 100km easily, though even basic SAM doctrine can limit their "Reach out and ruin someone's day" ability.
Meanwhile this entire situation is pathetic. The signal leak demonstrates that even Trump's admin knows that Europe and Israel can handle this themselves, which they insist we shouldn't pay for, and then burn money doing. We couldn't afford $100 billion a year in old, explicitly deprecated systems like Gulf War era Bradleys and weapon systems we stopped using because they aren't that great but are definitely better than nothing and CERTAINLY better than the trucks and golf carts Russia is using, but we can afford $150k per B2 flight hour to turn yet more of the desert into craters.