When you're using the -vs="1.5" command line switch together with catch all updates, will users with older versions than 1.5 be able to update to newest version?
No, they'll have no way to update to the latest version. The catch all update is for patch failures. (For instance, a user fiddles with a file, the patch update fails to apply, then the catch-all update is applied).
Considering to push this the start version up so we dont have to build so many (big) updates, but just force the oldest version to use the catch all update.
Incremental updates will solve most of this problem. This is coming in v2.6.20. We're working our hardest to get this out ASAP.