Starting with your last question: The USDA (United States Department of Agriculture) defines genetic modification as "heritable improvements [...] by genetic engineering or other more traditional methods". The EU defines GMOs as "an organism, in which the genetic material has been altered in a way that does not occur naturally" using DNA altering techniques.
These are rather legal than biological definitions. For example, the EU Court of Justice has recently ruled that organisms produced by mutagenesis are technically GMOs but do not fall under the GMO legislation.
So, let's look into how seedless fruits are produced:
This can be achieved by making a plant, that is able to grow fruits but fail to produce functional seeds due to unsuccessful meiosis. This is done for bananas and water melons by crossing a diploid with a tetraploid plant, which produces a triploid plant where chromosome pairing during meiosis is unlikely (more on this in the wiki article). For oranges this is done by growing genetic clones together. Since they cannot self-fertilize (and identical clones cannot fertilize each other), they don't develop seeds but still grow fruits.
These techniques don't require any genetic modification by targeted manipulation of the DNA. Crossing two plants with different characteristics is also done in conventional breeding, which is also not classified as GM. 667
However, it would be possible to genetically alter plants to block seed production and produce seedless GMOs. It would probably be a legal dispute whether this change is "heritable" when there are no seeds, so classification might depend on country policies.
The problem with seedless fruits is that you need seeds in order to grow new plants. To produce new seedless water melons you always have to cross the parents again. Plants that are able to reproduce vegetatively (parts of the plant can grow new plants), can be propagated by replanting branches. For plants, where this doesn't work very well, grafting is a solution to improve the process (a well growing plant is used as a root). Vegetative propagation in any case produces genetic clones, as you have pointed out correctly. The propagation method is independent of the technique that first produced the seedless plant and does not decide the GMO or non-GMO classification. 667