Sully,
Yes, the temp could be up more, but if the SG has been at .998 for that long, it's finished fermenting. I don't know why they would say .996 because lots of times fermentation will stop at a slightly highier SG. .998 is a common stopping place, too.
The main thing is that the SG is NOT changing. That tells you when fermentation is done. Your wine has been in the carboy for about 15 days and that is OK, but it is time to move to the next step.
Next time, try to keep the temp above 70 F, with a brew belt if necessary. Put the brew belt on when you pitch the yeast; take it off when fermentation is real active and the temp is above ~75. Then, when fermentation slows and the temp starts falling below 70, you can add the belt again. Most of us put the belt on carboys and have had no problems, so don't worry about it cracking.
When you are reading the SG, make sure you adjust the reading, based on the temperature of the must/wine. The hydrometer came with a chart that shows you how to adjust. Also, maybe you should test your hydrometer to make sure it is spot on. In 60 degree (check your chart to determine if your hydometer is calibrated for 60 or 68 and use that temp) distilled water, it should read 1.000. If it doesn't, adjust your readings by how much it is off and also by the temp of the must/wine.
Good luck.