sharing an experience

[Gnasher]

how much oil did you get out then without laying the bike on its side ? only some comes through those slots not all, unless the bike was pretty much flat on ts side . the benefit of taking the clutch cover off is i do know not all the oil comes through the slots ,you can see theres some below, it might if the bike was flat perhaps  , and you do need to know its all out.

You're confusing, sucking with pouring mate, it matters not about the casing lip.   If you lean the bike over as I've described and suck out via the the filler, you get it all out or as much as using the sump plug.  In fact if you tip the bike backwards slightly you get a little more than draining out the sump plug. 

Your issue with a buggered sump isn't a one off, I know of 2 bikes with the same issue.  One has rounded off the hex and utterly buggered whats remains trying to remove it with screwdriver and a hammer :rolleyes the only way now to remove without risking damaging the sump is to remove the sump.  The other is like yours stripped and he's bonded the sump plug in due to same issues of exhaust.  Both don't want to pay to have the sump removed, both have had several oil changes as mentioned above. 

This is only a 'get around', that said for many older type bikes the vacuum oil removal works fine, some newer engines and those with dual sump plugs it doesn't.