OK, I did some progress:
- re-downloaded everything - this time SHA-1 check was positive. Successfully extracted everything, no "scrambled" files anymore

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- resigned with ESSSE Resigner 4.4 eboot.bin, all SELF files, all SPRX files and one EDAT file (all of them as "3.55 disc boot", but also tried "4.21 disc too" - same effect. not tried "4.21 NPDRM free" yet), by using my RAP file + idps + act.dat . Funny fact - PSN Liberator 1.0 always ends in an error "failed to resign EBOOT.BIN", PSN Liberator 0.96B creates all folders etc., but neither of the files is resigned. So I had to do everything manually by using other resigner tools, primarily ESSSE Resigner (but TrueAncestor tools seem to work fine too).
- packaged the game as a disc game (changed param.sfo, directory name etc.)
- copied on external fat32 hdd
- multiman, webman etc. can perfectly see the game and load it in XMB
- if I start the game I get the error (after a while, not instantly) "Game seems to be corrupted, please reinstall it".
So my question now - is it one of such cases, where a game (in this case "Yakuza 5") can't be repackaged as a disc folder game? Would it be any different if I try to make it as an ISO? Do I really need to resign all files? What could be the cause for such a problem and a possible solution to it? Any help is hugely appreciated, I really want to finally solve this issue after having spent so many hours with it

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@aldostools Any ideas?

Many thanks in advance!!
One other idea/question - is any of the game managers (webman, multiman, iris etc.) maybe capable to load a game as a game folder (e.g. usb000/game/<game folder>) somehow directly into XMB, from an external HDD? in such case it would be at least a less elegant but a working solution too.
P.S. Is this tool maybe any good in my case
https://www.psxhax.com/threads/true...ler-v2-20-ps3-update-by-jjkkyu-released.2038/ ?
P.P.S. Does it matter which user account act.dat belongs to (on the same console) or is it OK to use it for any user account if on the same console? (this is my guess at least)