Agreed that Rose is out of comission rather than dead and gone and the majority of her is still there as Steven/her & steven's gem. I think the Gems have no clue if/how Rose as Rose will ever return apart from previously unknowns appearing such as Lion.
I've enjoyed the gems (including Steven) coming to terms with Rose as they knew her seeminlgy gone permanently in exchange for Steven. Steven references this in Joy RIde: "I think they kind of blame me for my mom not being around"
Lion is more than a simple cat bag or Rose could have stored her stuff inside her/steven's gem or her room plus gems can make powerful seperate entities from themselves (Pearl, Lapis) In Lion's case I think a part of Rose's conciousness is there alongside her storaged stuff. Yes gem monsters come with gem shards but still Lion fits nicely as a similar offshoot of gemkind.