I am starting to see.
As hinted before it seems like the good Acronis guys are NOT making a "standard" CD.
UltraISO is reknown (to me at least ) to do some non-standard things as well (often, additionally not particularly well documented) .
In the 2012 iso image I have handy, the situation is as follows (and I believe the ones you have are not that much different).
There is actually a "hidden" volume image (FAT16, making use of the proprietary Acronis BOOTWIZ bootmanager bootsector) that extends from sector LBA 52 (2048 bytes sector) for an extent of 194397 sectors (in 512 bytes sector).
The actual Boot Catalog is on sector LBA 39 (and does NOT "index" this image).
Sectors LBA 40-51 contain binary data (possibly a loader of some kind, I'll have a look at it), which is the file that both UltraIso and Isobuster see as "Acronis.img".
The issue with a No-emulation entry in Boot Catalog is that it provides NOT the extension (size) of the image, but rather the number of Sectors to Load (usually 4, and that is the reason why originally the bextract.cmd batch was written http://reboot.pro/to...ting-iso-files/ ), all utilities will "see" (and incorrectly show as "size") only the "size to be loaded".
The first "next" LBA sector actually indexed in the Boot Catalog is the "next" entry (the EFI one) on LBA 48652.
What Ultraiso extracts as .bif is sized 187,869,184 bytes,i.e. 187,869,184/2048=91,733 (2048 bytes) sectors.
The actual .bif then comprises BOTH the "LARGE El-Torito No Emulation image" i.e. is the extents 40-48651 that are made of:
In 2048 bytes sectors:
Loader 12 sectors
FAT 16 image 194397/4= 48599.25 rounded to 48600
Some "mystery" sectors <- (need to double check)
AND the whole extents 48652-91773 of the "EFI" image (that UltraIso *somehow* does not "detect/highlight" properly in the sense of "separating it from the first", it exports the .bif containing "all it can find").
91773-40=91733 which is the size of the .bif, Q.E.D.
So, if you mount in IMDISK the .bif, once with image file offset 12*4=48 (512 bytes block) you get access to what you call "BIOS.IMG", while if you mount the same .bif with image offset (48652-40)*4=194448 (512 bytes block) you get access to the "EFI.IMG"
You will need to replace the 40 and 48652 in the above calculation with your values, and as well it is possible that the size of the loader is not in your .iso's 12 sectors, but once adapted it should work on your images as well.
It is still not clear at all (to me) how can UltraIso later "show" the contents of the .bif (which actually are the contents of the first FAT image, most probably it simply scans the file to find a recognizable BPB :unsure., I'll have to check with some dummy data.
About the little batch, if we put a "limit" (using common sense) to the possible size of the loader (let's say between 1 and 20 x 2048 bytes sectors) i.e. to the extents of the "search" for the bootsectors/BPB that needs to be performed, and we assume that the image is FAT formatted, it will be easy to find it and extract the corresponding sectors.
Still it has to be seen what the "mystery sectors" are related to.