It is certainly possible that the Lady and Ariel are the same being for many of the reasons stated above but I think the theory could be pushed further.

Quick Note:
(What follows is naturally, simply an opinion based upon the accumulated reading of a decade or two, I have no idea how closely it would fit with the official background but it is how I make sense of it all, at least most of the time. )

It is also possible to extend the web even further, of the known gods and goddesses of the Empire, for example, Rhya seems a perfect fit as well. Worshipped in conjunction with Taal in much the same way as Ariel and Orion are linked; she is also a nature goddess with dominion over life and fertility. And the three goddesses share direct or general thematic links to Isha. Even the dwarfs developed a protector goddesses in Valaya. The common themes of stability and growth can be found within each I think.

Now this may simply reflect the general popularity of mother goddesses in the mythologies that served as the source material that inspired GW and the similarity may end there.

However, I think that the development of a nurturing protector goddess in the major cultures of the Warhammer World signifies a desire (even if merely unconscious) to combat the warping impact of chaos. As each deity has a more militaristic side when confronted directly by the warping properties of chaos; for example even followers of Shallya (perhaps one of the most peaceful dieties in the Warhammer World) react viciously to followers of Nurgle.

The way I tend to view Ariel/The Lady/Rhya/Isha/Valaya is as a single concept (the desire of the natural world to find balance/ the rejection of chaos as disruptive) with many forms (human, dwarf, elf, giant moth and so on). In this way they can be both separate and as one. Each group has an impact on the form of the concept and on the methods it uses to achieve its goals. The expectations of the worshippers provides the structure needed for the goddesses to interact with the physical world. At the same time the fundamental concept remains unchanged and is an expression of the revulsion felt by all natural creatures for the works of chaos.

Put more simply, if the Chaos Gods are a virus attacking the Warhammer World then the protector/nature goddesses are a kind of antibody, they all spring from the same source (the sentient life of the planet) but are adapted to their particular environments and common opponents.

regards,
IT