I am going to assume that Kale is a plant derived from wild cabbage and has been selected upon for certain attributes by humans for 1000's of generations.
1 There are two principle reasons why genetic (allelic) diversity might be lower in the kale subgroup than in the wild cabbage. Firstly, strong selection imposed by humans may have diminished allelic diversity. Strong selection can drive alleles to fixation. Secondly, if the kale has come from a small sub-population of the wild cabbage it may have been through a genetic bottleneck, this is likely to reduce allelic diversity because, unless all alleles from the original population are captured in the founder populations, some alleles will be lost by genetic drift.
2 Generally allelic diversity is a "good" thing in an unstable environment. It allows different trait combinations and variation in traits to occur. If the environment changes drastically then it could impose severe selective forces on the population, those with high allelic diversity are likely to come up with a combination of alleles that survives.
E.g. Humans only allow the cabbages over 18cm in diameter to reproduce in to the next generation. 5 loci affect cabbage size, each having a "large cabbage" (C) allele and a "small cabbage" (c) allele. Having 5 c alleles gives a cabbage size of 15cm. Each C allele adds 1cm. If a population has fixed the c allele at 3 loci (i.e. there is low genetic diversity) then none of the cabbages will be >18cm. If a population has both the C and c allele at all loci then some of the population will be over 18cm.
3 The final question is difficult to answer, it depends on what you define as "a good recommendation," that is very subjective. If the aim is to maximize allelic diversity then yes, if it is to maximize cabbage size it is probably no (because the "genetically engineered" plants have been selected upon for large cabbages - they might have the C alleles fixed in the populations!)