"Unfortunately, the bolt head rounded off and now I'm trying to use a 'bolt out'.... without success"

What bolt head rounded off? One of the 10mms, or the 30mm?

It is unlikely that you have damaged any valves. Regardless of whether you have or not, I would proceed under the assuption that you have not damaged the valves.

You should not (and do not have to) remove the crank pulley in order to change the timing belt. I repeat, DO NOT REMOVE THE crank pulley.

Now, knowing that you do not have to remove the crank pulley, you are going to have to stare and probe at the gaps and slots around the crank pulley to figure out how you are going to thread the new timing belt around the crank pully. Maybe practice with a piece of the old timing belt.

It can be done--'cause I and a million other people have done it.

As far as the bolt the you've rounded-off, it may, or may not, be important. Was it tightened (or re-tightened) before you rounded it off? If the bolt was tightened (or retightened) don't worry about the rounded head. If it is not tight, you are going to have to replace the bolt, but Forget about the easy-out-f-you are asking for far more trouble if you break off an easy-out in the bolt (assuming you do have to remove the bolt).