Hi,
So I backed up my data and ran fsck.
fsck didn't give any error when it ended (sorry I forgot to get a copy of it to paste here) but it wasn't clear if it had ended up well either..
So I ran these commands again:
grub> insmod fat
grub> set root=(hd1,7)
grub> linux /vmlinuz-4.9-x86_64 root=/dev/sdb8 rw
And got this message:
error: premature end of file /vmlinuz-4.9-x86_64.
This looks to me that fsck couldn't fix the issue with the partition.
I went back to livecd to have a look at the partition I tried to fix and it looked like that:
[manjaro@manjaro-cinnamon C078-4715]$ ls -al
total 33968
drwxr-xr-x 2 manjaro manjaro 4096 Mar 31 13:36 '$RECYCLE.BIN'
drwxr-xr-x 6 manjaro manjaro 4096 Jan 1 1970 .
drwxr-x---+ 3 root root 60 Aug 5 00:57 ..
-rw-r--r-- 1 manjaro manjaro 4907008 Jan 1 1980 FSCK0000.REC
-rw-r--r-- 1 manjaro manjaro 4096 Jan 1 1980 FSCK0001.REC
-rw-r--r-- 1 manjaro manjaro 6328320 Jan 1 1980 FSCK0002.REC
-rw-r--r-- 1 manjaro manjaro 22532096 Jan 1 1980 FSCK0003.REC
drwxr-xr-x 2 manjaro manjaro 4096 Mar 31 13:34 'System Volume Information'
drwxr-xr-x 6 manjaro manjaro 4096 Jul 31 22:45 grub
-rw-r--r-- 1 manjaro manjaro 0 Jun 26 22:41 initramfs-4.9-x86_64-fallback.img
-rw-r--r-- 1 manjaro manjaro 0 Jun 26 22:41 initramfs-4.9-x86_64.img
-rw-r--r-- 1 manjaro manjaro 987648 May 13 09:38 intel-ucode.img
-rw-r--r-- 1 manjaro manjaro 0 Jun 24 07:08 linux49-x86_64.kver
drwxr-xr-x 2 manjaro manjaro 4096 Aug 3 2016 memtest86+
-rw-r--r-- 1 manjaro manjaro 0 Jun 24 07:08 vmlinuz-4.9-x86_64
These FSCK000x.REC files would be what fsck managed to recover so I thought I could rename them to whatever they were previously and see if I had more luck...
At this point the option was to reinstall and I could spare a little more time.
So here's what I've done:
[manjaro@manjaro-cinnamon C078-4715]$ file FSCK0000.REC
FSCK0000.REC: Linux kernel x86 boot executable bzImage, version 4.9.35-1-MANJARO (builduser@manjaro) #1 SMP PREEMPT Fri Jun 30 06:42:12 UTC 2017, RO-rootFS, swap_dev 0x4, Normal VGA
[manjaro@manjaro-cinnamon C078-4715]$ file FSCK0001.REC
FSCK0001.REC: data
[manjaro@manjaro-cinnamon C078-4715]$ file FSCK0002.REC
FSCK0002.REC: gzip compressed data, last modified: Wed Jul 5 20:21:18 2017, from Unix
[manjaro@manjaro-cinnamon C078-4715]$ file FSCK0003.REC
FSCK0003.REC: gzip compressed data, last modified: Wed Jul 5 20:21:22 2017, from Unix
[manjaro@manjaro-cinnamon C078-4715]$ ls grub/
fonts grub.cfg grubenv i386-pc locale themes
[manjaro@manjaro-cinnamon C078-4715]$ mv initramfs-4.9-x86_64-fallback.img BKP_initramfs-4.9-x86_64-fallback.img
[manjaro@manjaro-cinnamon C078-4715]$ mv initramfs-4.9-x86_64.img BKP_initramfs-4.9-x86_64.img
[manjaro@manjaro-cinnamon C078-4715]$ mv linux49-x86_64.kver BKP_linux49-x86_64.kver
[manjaro@manjaro-cinnamon C078-4715]$ mv vmlinuz-4.9-x86_64 BKP_vmlinuz-4.9-x86_64
[manjaro@manjaro-cinnamon C078-4715]$ ls -al
total 33968
drwxr-xr-x 2 manjaro manjaro 4096 Mar 31 13:36 '$RECYCLE.BIN'
drwxr-xr-x 6 manjaro manjaro 4096 Aug 5 01:08 .
drwxr-x---+ 3 root root 60 Aug 5 00:57 ..
-rw-r--r-- 1 manjaro manjaro 0 Jun 26 22:41 BKP_initramfs-4.9-x86_64-fallback.img
-rw-r--r-- 1 manjaro manjaro 0 Jun 26 22:41 BKP_initramfs-4.9-x86_64.img
-rw-r--r-- 1 manjaro manjaro 0 Jun 24 07:08 BKP_linux49-x86_64.kver
-rw-r--r-- 1 manjaro manjaro 0 Jun 24 07:08 BKP_vmlinuz-4.9-x86_64
-rw-r--r-- 1 manjaro manjaro 4907008 Jan 1 1980 FSCK0000.REC
-rw-r--r-- 1 manjaro manjaro 4096 Jan 1 1980 FSCK0001.REC
-rw-r--r-- 1 manjaro manjaro 6328320 Jan 1 1980 FSCK0002.REC
-rw-r--r-- 1 manjaro manjaro 22532096 Jan 1 1980 FSCK0003.REC
drwxr-xr-x 2 manjaro manjaro 4096 Mar 31 13:34 'System Volume Information'
drwxr-xr-x 6 manjaro manjaro 4096 Jul 31 22:45 grub
-rw-r--r-- 1 manjaro manjaro 987648 May 13 09:38 intel-ucode.img
drwxr-xr-x 2 manjaro manjaro 4096 Aug 3 2016 memtest86+
[manjaro@manjaro-cinnamon C078-4715]$ mv FSCK0000.REC vmlinuz-4.9-x86_64
[manjaro@manjaro-cinnamon C078-4715]$ mv FSCK0003.REC initramfs-4.9-x86_64-fallback.img
[manjaro@manjaro-cinnamon C078-4715]$ mv FSCK0002.REC initramfs-4.9-x86_64.img
[manjaro@manjaro-cinnamon C078-4715]$ ls -al
total 33968
drwxr-xr-x 2 manjaro manjaro 4096 Mar 31 13:36 '$RECYCLE.BIN'
drwxr-xr-x 6 manjaro manjaro 4096 Aug 5 01:13 .
drwxr-x---+ 3 root root 60 Aug 5 00:57 ..
-rw-r--r-- 1 manjaro manjaro 0 Jun 26 22:41 BKP_initramfs-4.9-x86_64-fallback.img
-rw-r--r-- 1 manjaro manjaro 0 Jun 26 22:41 BKP_initramfs-4.9-x86_64.img
-rw-r--r-- 1 manjaro manjaro 0 Jun 24 07:08 BKP_linux49-x86_64.kver
-rw-r--r-- 1 manjaro manjaro 0 Jun 24 07:08 BKP_vmlinuz-4.9-x86_64
-rw-r--r-- 1 manjaro manjaro 4096 Jan 1 1980 FSCK0001.REC
drwxr-xr-x 2 manjaro manjaro 4096 Mar 31 13:34 'System Volume Information'
drwxr-xr-x 6 manjaro manjaro 4096 Jul 31 22:45 grub
-rw-r--r-- 1 manjaro manjaro 22532096 Jan 1 1980 initramfs-4.9-x86_64-fallback.img
-rw-r--r-- 1 manjaro manjaro 6328320 Jan 1 1980 initramfs-4.9-x86_64.img
-rw-r--r-- 1 manjaro manjaro 987648 May 13 09:38 intel-ucode.img
drwxr-xr-x 2 manjaro manjaro 4096 Aug 3 2016 memtest86+
-rw-r--r-- 1 manjaro manjaro 4907008 Jan 1 1980 vmlinuz-4.9-x86_64
I did some file size matching looking at a manjaro install I have on another laptop.
As you can see there is one file I haven't renamed, I don't know what it was, from the size it looks like it could be a directory but no idea what it could be.
After doing that I ran these commands again:
This time no error and it could boot ...until it stopped complaining about /dev/sdb8 that couldn't be found or mounted and it throw me into some strange shell ([rootfs ] if I remember well).
(Sorry I don't have more detail about the error but I didn't feel like writing it down and my memory has its limits..)
Anyway, I had noticed earlier that the livecd had an option to boot to the manjaro install it had found on hd1,8 and I thought I could give this a try (I had tried it before my fsck experiments and it had failed like it did from the grub shell).
This brought me to the grub menu and I could boot successfully!

I am pretty happy about this. Thank you very much for your help @gohlip
I am now updating the system, I hope it was not a stupid idea.
Then I will try another reboot before I can go happily to bed.
The next step is to get the win10 stuff back into my grub menu ..or maybe I will wait a bit for that.
Expect more questions from my side some day.
Cheers