Yes to scenario 1
The Help action says
Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally’s Attack more effective. If your ally attacks the target before your next turn, the first Attack roll is made with advantage.
The Ready action says
First, you decide what perceivable circumstance will trigger your Reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include “If the Cultist steps on the trapdoor, I’ll pull the lever that opens it,” and “If the Goblin steps next to me, I move away.”
You can certainly perceive an enemy getting attacked, and there is no reason why readying an Help action wouldn’t work.
Also yes to scenario 2
Both Help and Ready are actions that familiars are allowed to take, so the second scenario works mostly the same as the first, except that now you’re casting a spell instead of taking multiple attacks. Eldritch Blast is instantaneous, but with spells involving multiple attacks apparently these are not simultaneous according to this Q&A, so there’s no reason why reactions couldn’t take place between multiple blasts.
But it might not work as well as you hope
Because you only actually take the action when the trigger occurs, the creature you’re helping to attack needs to be within 5 feet of you after the first attack hits. If the creature you’re trying to help attack gets a turn in which it decides to move, or is moved through some other effect, you won’t be able to use your readied Help and might’ve wasted your reaction, so while it may make sense to do this for your familiar it very rarely makes sense for a PC to do this unless you have specific features that enhance Help like the Mastermind.