Yellowing of tomatoes can be caused by a variety of things. Usually viruses show up with other symptoms such as distorted foliage, mottling (rather than just yellow) and total deformity. Some heirloom tomato varieties have what is called potato leaves, that is they are bigger and oftentimes more coarse. I'm not sure the slight curling you describe is out of the norm. Heirloom tomatoes are, by nature, quite disease prone compared to hybrids. What comes to mind are diseases such as Verticillium and Fusarium wilts. These can cause a yellowing of the leaves and death of entire branches. Unfortunately there is no control for them. Yellowing can also be caused by watering and/or nutrition issues, but it sounds like you have addressed these by fertilizing, and (I'm sure), maintaining even moisture. The issue of no flowers or few few can be related to temperature. Any daytime temperatures over 90 degrees F, or nighttime temps in the 80's can prevent fruit set, so that would be something to look at. At this point I think your approach is to keep the plant watered, feed regularly (about every 2 weeks), and try to maintain health. If yellowing occurs, continue to remove the affected branches. If you have a reliable local garden center or extension office, you might consider taking the yellowed stems to them for a diagnosis, as they may be able to see something.