People in the scrum team are representing stakeholders

project lead

Are you a product or project focussed organization?

as this would be too much for a product owner to handle

One product is owned by one PO. Even though one person can serve multiple PO roles (so serve multiple products), this type of scaling is problematic. So sure enough, it would be hard to handle for one person.

Did you consider not burdening these tasks into the development team, but have more PO-persons?

But since you do have a PO, these project leads are by definition part of the Development Team!!!

Now what happens because of this is that the project leads go directly to the team and bypass the PO to have epics/user stories refined and then hand the refined items over to the product owner.

This is really REALLY iffy if you ask me. They do not go to the team, because they are already part of the development team. Why are refined stories handed back to the PO? What does this whole situation do for your effectiveness and transparency? How can you ensure maximizing value?

He receives an epic/user story, and when he finds it important he asks the team to refine it.

Receives? Huh? He creates PBI's (or let them be created on his behalf), and the teams refine them based on the Product Backlog ordering to have enough work Ready to meet the Sprint Goal you mean?

What do you think, should the product owner be bypassed?

What do you think? How does bypassing the PO help your transparency and maximizing value?

How should we handle 1 backlog with different products?

What does the Scrum Guide say about this (hint hint ;)?

This is from the Scrum Guide section that describes the Product Owner role:

The Product Owner is the sole person responsible for managing the Product Backlog. Product Backlog management includes:

• Clearly expressing Product Backlog items;
• Ordering the items in the Product Backlog to best achieve goals and missions;
• Optimizing the value of the work the Development Team performs;
• Ensuring that the Product Backlog is visible, transparent, and clear to all, and shows what the Scrum Team will work on next; and,
• Ensuring the Development Team understands items in the Product Backlog to the level needed.

The Product Owner may do the above work, or have the Development Team do it. However, the Product Owner remains accountable.

The last sentence comes into play with your scenario.  I see nothing wrong with the way items are being handled as long as the Product Owner remains ultimately accountable for the Product Backlog.

The real issue I have with your scenario is that you do not have a Product Backlog.  You have a long list of things that need to be done to a lot of different applications and one group of people that does the work by creating knowledge silos. You asked 

How should we handle 1 backlog with different products?

My answer is break it up into multiple backlogs.  Or you might want to look at how you are defining products.  It could be that you only have 1 product with multiple components. Make sure you are actually talking about different products. In what ever case you find yourself, remember passage from the Scrum Guide above and the fact that the Product Owner is ultimately accountable for the bullet list items.

In our scrum team we work on several products (+- 8), of which some are created by ourselves.

Per product one person functions as project lead and represents the stakeholders, he/she
- tells the product owner what to add to the backlog,
- keeps track of all the deadlines and
- does external communication

as this would be too much for a product owner to handle

From your reading of the Scrum Guide, what do you think a Product Owner *should* be in a position to handle?