Ok: after one month with a fully working 10.6.7 (except for the mic, but I don't use it) — by the way 10.6.8 sleep fix works on 10.6.7 as well ;-), I've tried again (still on my external disk, though it's probably an issue...).
Here's the final process I took, based on what was written here:
1) directly installed 10.6.8 Combo Updater (I had used Netbook...2351 for my former working 10.6.7, so it was not needed again); Repairing Disk Permissions is not indispensable but do it if you've got an old system
2) reboot ok; NBI did its job; I had not exactly a reboot "loop" because I was on the external disk, but kind of (it rebooted on the internal one)
3) still no way to boot in safe mode on the external disk, but succeeded doing code tricks from internal disk (caution: use the full paths for file names in this case! I had to do it from root user so sudo was not needed either)
4) this time sound output is ok, no mic (I had tried it yesterday but the interface wasn't even detected... so it looks that it's not always working, who knows why)
=> I leave it this way for now...
N.B.: in my former unsuccessful trial I had tried to install legacy_kernel-10.8.0.v2.pkg which is included in the 10.6.8 Kit but it just screwed everything and I couldn't correct things back. It doesn't seem to be needed (or I don't get everything, which is possible ;-D)
N.B.2: while the thing was screwed, I tried VoodooHDA to fix audio and it worked (even for the mic!), but with a huge hiss on both audio in and out... (the system was working ok, but I had an error message at startup — something about 64bit unsupported — I preferred doing the whole thing from scratch to be sure.
N.B.3: try to backup your working system everytime you're trying something, it's a lot faster to recover from a disk image than reinstall everything! ;-)