I'm not sure I understand what happened to you here. I have "offline" (which isn't truly offline at the moment) and online characters, and have separate stash sets for both. No, stashes can't be used between the two types of gameplay, but I've never had my stash disappear on me or be lost.BEWARE about offline characters! There's a mode that used to be called Solo Challenge. The text (at least during early access) says that you share your stash. You do not, in fact, share your stash. I had HUNDREDS of thousands of gold tied up in stash tabs (you can have like 100 of them) all color coded and set up for ease of use. I have 75+ levels of my main just filling it up so I could help new characters level. Nope. I had to change his mode and lost everything to share a stash. I know it's my fault to not look into it more before I did it, but the tool tip specifically stated you could share between characters in that mode.
Right now all characters are on EHG's servers and are technically online. There will be an actual offline mode with 1.0.