If you are worried about cross pollination of your apple trees, then you can buy a 'family tree'. Then you don't need to worry if there are suitable trees around you to pollinate your own tree.
A Family Apple tree is basically a single rootstock, onto which three different varieties that are guaranteed to cross pollinate each other have been grafted - ensuring a good crop of each variety.
You can have desert and cooking apples on the same family tree! Family Trees are particularly suited to small gardens where there is only room for one tree.
Family Apple Trees are normally grafted onto a semi-dwarfing root stock - such as M26. This ensures that they are suitable for small gardens. The normally grow to a maximum of 7 - 9 feet tall (3m). The ultimate size will be determined by the way in which you prune your Family Apple Tree.
One potential problem with Family Trees, is the fact that one of the varieties can become the dominant partner, at the expense of the other two. (A bit like life really!). Just keep an eye on this situation, and adjust your pruning to ensure a well balanced Family Apple Tree.
There is a temptation to try and grow Family trees in containers. This is all very well, but the same rules apply to tree on dwarfing rootstocks as any other types. They will need to have a good supply of nutrients to bear fruit. It is not easy to do this in containers. Reasons being, lack of regular nutrient and occasional drying out of pot, leading to nutrient lock-out.
If you are going to try your luck with a Family Apple Tree - and why not? - then plant it in the ground where it will not be subject to sudden changes in environment at and below ground level.
Container growing fruit trees is not a bad idea, but it only needs one mistake insofar as watering is concerned, to spoil you entire crop. Then you have tom wait a further year, to start again.
Another option, is to buy a 'double' tree. These are generally better balanced without too many problems.
Uneven growth, or even gradual die-back of one portion is not uncommon. One of the reasons for this is a failure of one of the grafts at 'propagation' time. As with all things, the stronger will survive and the weakest will fail.
Often not realised with family trees is that some varieties on the tree will be happier in your given environment than others. It only needs a small shift of power, for the uneven growth to materialse.