I think everyone has screwed up mash temperatures at some point and got decently drinkable beer out of it. If your Oktoberfest is mostly base malts (ie no crazy amounts of crystal or caramunich in there) then that works in your favour.
IT'll be a little sweeter than you intended, but given the style, personally - I wouldn't mind. Hopefully you took notes on your strike water and amount of grain so you can learn from your "mistake" but I'm confident you'll enjoy your beer. But, you should have brewed this style in late July/early August to be more timely
Sorry I don't have a link, brulisopher did an exbeerimwnt with extreme varied mash temps, and while the beers were significantly different FG, many tasters couldn't tell the difference..
Sugar additions raise the OG but will have no effect on the FG.
In an D/IIPA, you use simple sugar in lieu of a portion of malt. This would be in addition.
I recently brewed a batch of OktoberFAST. I mashed a little higher than intended, 158F. I hit my OG as expected, 1.051, but my FG was 1.024. While I was disappointed when looking at the hydrometer, the beer came out great. I was expecting it to be rather sweet, but surprisingly it wasn't. I think the style has a lot of latitude so you'll be alright.