[General Questions and Answers] HisDudeness3008:
Hiya! I don't believe my problem is device specific. The background of how I ended up in this crappy situation is, but I believe the resulting issue is general. Should I be wrong, tell me and I'll move this to my device's section.

Short question: how can I (and can I?) restore data in a partition that got deleted, if no new filesystem has been created over it?

Long background: I have a Xiaomi Mi2-S 32GB. It used to have a peculiar layout: a double system partition (/system1 and /system2)¹, a small internal storage (/userdata)², and a big emulated SD card (/storage)³.

Let's explain why:

¹ False dual boot: the active system is installed in the first partition. When installing an update with the official app, the newer system gets installed in the second and that one gets booted. So, should this newer system fail to boot, you have an older one correctly working and ready to boot.

² and ³: so that the whole storage partition containing photos, music, videos, downloads, backups, saved games and such can be accessed with MTP, while the userdata containing apps and complementary system things is kept safe. This last decision, however, brings up a new problem: userdata can't be accessed by user to put his files or by apps without root permissions to store data (like photos), while storage can't be used to install apps, or even to move them using Link2SD or such. Some users might find storage is insufficient for their videos and music, while others might find userdata is too little for their games, and they are both stuck in this situation.

I was in the second group, so I altered my layout using stillka's guide on xiaomi.eu (Sorry, I can't post links). I extended my userdata, so that my storage resulted smaller. Plus, I understood altering a partition would mean deleting all the partitions before that one, and recreating them thereafter.

Until this point, all was OK. I installed Ivan's AOSP Lollipop for unmerged partitions, and found out it would experience random reboots with True Dual Boot. So I stuck with False one and forgot about everything. I kept that version without updating for a long time.

Then, several months later, my phone started rebooting randomly anyway. I figured I would come back to MIUI to get Xiaomi's support for an official ROM.

Little did I know they decided to change layout in the meantime. MIUI got so big the size of the two systems was insufficient. So they decided to merge them into an unique partition big enough. So, while flashing with the official tool MiFlash, it practically altered my system layout, having to delete all that was placed before them (cache, userdata and storage), never telling me what it was going to do, advising me to back my storage up somewhere. All I did was back up my userdata into storage, confident flashing their official ROM with their official tool would just write into system, since nobody told me otherwise.

So this is the result: the old, small size of userdata is back, and everything that comes after is left without any filesystem: these are the last line in parted's print output

I've tried parted's rescue command, but it is unable to find a partition lying there. I don't have my old layout, so I'm not able to precisely know where my old storage began, but I remember it to be around 18 GiB in size. I've tried all ranges possible (from the current end of userdata, 18 G from the end and so on) but no dice.

Can someone tell me if there is any hope, and what can I try?