She doesn't actually need to get engaged to Evan.
Evan and Serena just need to be close and the rumour mill will do its job.
Royalties are usually difficult to approach because of their rank, and their cautiousness because all relationship have political value.
If Evan is letting Serena close, and she doesn't refuse...
- Evan is interested in Serena and is in process to court her.
- For those with view on the bigger picture, they would notice the motivation could be linked to Einrich getting engaged to Rosemary and securing support from the Violet. Evan's motivation can be seen as trying to get Serena as a balancer.
Either way, be it romantic or political, public will see Violets is supporting both sides, so the other nobles shouldn't act prematurely.
Keeping it as friends are also smart. There's no obligation in it.
Einrich and Rosemary are engaged. That means there is a formal agreement between both families. It can't be called off by one side without any repercussion unless you have a really good reason for it. Even then, it must go through the head of the family because for the nobilities, marriage are all political. Love is a bonus rather than requirement.
Meanwhile, Evan and Serena can always call it off when they like, because they are just friends. Family is not involved at all, and they're not responsible about what outsiders assumed of their relationship.