It's very common for gable-type venting to be the only venting in a roof. Unfortunately, the shingling on these types of roofs don't last as long as those with soffit vents. At the same time, when and if you have the roof re-done, have a ridge vent installed, end-to-end. This will help draw air out should you decide, sooner or later, to put in soffit venting. Your call.
Now, as for the bump in the roof, this is usually caused by an expanding roof. Personally, I haven't seen a bump due to an over-heated roof in a home. Usually, at some point, water has gotten to the sheathing and this caused the bump. Seeing it in the heat was, well, kinda an after-though if you know what I mean. Anyway, do check for water rings or dried water on the under-side of the bump. If you have to re-do the roof, go right down to the sheathing to see what you have. Then you can replace any bad sheets.