Is the shared stash on single player safe

I've had 0 issues with any stash in any mode, even though the overflow thing does exist. Though as far as I recall, that was applicable to char saves rather than the stash save on SP as those kept the old limits despite increased capacity. Might be misremembering that though.. Either way, it won't affect most people as it really only hits when there's too much affix information and such to store, meaning when there's a crapton of fully-variable items in there (e.g. nothing but rare rings).

Either way, the beauty of (at least PC) SP is that you can protect (or clone *cough*) whatever you want to protect, simply by creating a backup:
- Hit up your d2:r save game folder (typically C:\Users\YourUserNameHere\Saved Games\Diablo II Resurrected)
- Create a backup of the charactername.d2s file for whichever character you'd like to protect
- Create a backup of SharedStashSoftCoreV2.d2i to protect your shared stash

If you then do something that ends up breaking things somehow, you always have a fallback to drop back to, simply by copying over the previous version.