In principle, during pollen and egg cell formation and in the resulting seed the same processes can take place as during vegetative propagation. Usually two parents are involved, but in self-fertile genera like Hosta it can be just one. Moreover most plants don't come true from seed unlike cuttings. The explanation for the observed variation therefore is a bit different. So there is always seed involved.
Mutation has nothing to do with it.
Only crossing two wild species will give more or less uniform offspring in the first generation. Chimaeral rearrangement can take place in the germline if the mutation is in chloroplast in a single cell. A nuclear mutation can only lead to a chimaera directly if happening at the stage that the organism already has two or more cells. Meiotic recombination occurs during formation of pollen and egg cells with a high frequency. So, if the parents are not homozygous, i.e. , have several genes in two different forms, this may lead to new combinations in the offspring due to segregation.