Hiya.
Yes, you are right, all offers you make to the AI are accepted. As you say in real life, not always the case. This is unfortunately a limitation of the SIM. You as the receiving sector can amend the offers from the AI btw.
Traffic can really be that low initially. FL280/300 happens for heavy traffic out of Amsterdam or Brussels. Unfortunately another limitation of the sim is you can only enter one RFL in the flightplan. Not a request for higher at a certain waypoint as in real life. Example: in real life they would file initially 280, then 300 at SAM, then 320 at CRK. But not possible in the sim. I look at it as an interesting challenge. It means that you have to climb/descent traffic through the 280-320 cruising traffic.
Happy controlling, C