IIRC Eurowings is a Lufthansa regional offshoot which effectively flies as LH (think Qantaslink affiliates), and so their flights should be considered LH metal. But do ask the M&M call center to be sure (skype the ones in Switzerland or Germany for best results... the Melbourne call center is pretty sketchy).
SIN-CGK-SIN SQ F runs were popular with people doing SQ PPS sector runs under the old PPS sector system. I seem to recall that the turnaround was relatively straightforward. Do a search on the SQ forum on FT or SQTalk. This would date back to 2006/7 though.
edit: Found this in the LH forum: SIN to CGK vv on LH [merged Jakarta Q & A thread] - FlyerTalk Forums
A more up to date post involving CX corroborates the above: SIN to CGK vv on LH [merged Jakarta Q & A thread] - FlyerTalk Forums
Thanks for that - it worked out cheaper, spending the night in Jakarta so dont have to worry now
Now I have booked my 2 segments - i wonder if I have to fly them for my status to show up etc - when i log in - doesnt appear to be any different than before...