I agree with your comment re the viability of a HEL route but as they are now giving OSL a try, it might work.
As for the Air Malta RJ70s, DUS was probably a bad example. The RJ70s were purchased by Air Malta to provide services from smaller regional airports inaccessible for larger aircraft, thus making Malta an anttractice holiday destination for the local folks. For example, in Germany airports such as BWE, KSF and HOQ were served, all with 1.500m runways. BWE-MLA, for example, is 1152mi, KSF-MLA 1.103mi. Not sure how much the difference in runway length will eat up from that perimeter, but it might be partially offset by the fact that, for example, KSF has a field elevation of 977ft. I would certainly be interested to find out what the max range of the RJ70"ER" with a sensible load out of LCY is, particularly as two appear to be up for grabs.
I just had a look into the RJ70 issue - 12 have been built, one has since crashed (1254, 1258, 1260 and 1267 are of the ER subtype)
1223 SE-DJX Transwede opf AirOne
1224 SE-DJY Transwede
1225 SE-DJZ Transwede opf AirOne
1228 VH-NJT National Jets
1229 G-BUFI MDLR Airlines (but apparently stored in the UK)
1230 VT-MDM MDLR Airlines
1252 available *ex Turkish
1254 EI-COQ Transwede opf SAS
1258 available *ex Euromanx
1260 available *ex Euromanx
1267 A6-RJK Royal Jet
Not sure about SEDJY. As the GOA TRN flights appear to have fallen through, it should be available for SAS.