Lots of interesting stuff guys.
OP ... quiet type eh ?
Ummmm still did think of junk to add, no idea what's wrong with your grub entry but yeah, doesn't look right regardless. Check out this https://forums.bunsenlabs.org/viewtopic.php?id=6294
has an example of a working Bunsenlabs grub entry, which is based on Debian stable itself. Also am sure there's other possible useful junk stated on the topic in general. Pretty sure mentioned digging through the iso's to find out what's, where and that needs to be used to find a working entry. Also fairly sure mentioned a person avail themselves of existing tools example menu entries and the grub.cfg's they share ... I mean in ways a usb drive is like whatever other type block device(s.) Still use that multiboot thumb drive, still a bunch of iso's havent figured out the mystic incantations to get to work yet.
Also clarification, I got a Debian iso to boot from hdd, never completed an install from it. Remember this thing can be a real pita and very frustrating. Someone just has to keep at it, stay persistent and eventually will probably unlock the magic words for the right entry to boot whichever iso's. Good luck.
Ps, also wanted to point out things like PXE installing for whoever. Sounds like OP has more than one pc onhand. So again .. unless it's just a hey I want to do this, like almost always in gnu/Linux, more than one option to attain techno-bliss.