Ahhh, this old chestnut. The story is quite simple, but a sort of Chinese-whispers effect has led to some confusion over the years.
That image is from the MTDP from around 2003. The MTDPs only show loose ideas that the park have at the time. That drawing, for example, shows the top hat being the other side of the Stealth area. The layouts mean nothing; they're just there as a rough indication of size.
When Thorpe were drawing up ideas for Stealth, they wanted a longer ride, more akin to Xcelerator (hence why the MTDP shows that). Ultimately, it never happened, for a variety of reasons: space, money, visual impact of the ride, etc. There is no online copy of any longer layout they may have considered.
The "Stealth and Rita were meant to be one ride" thing is one of these 'fun facts' that the parks liked to say during behind the scene tours. However, that's very much an embellishment of the truth. Thorpe had long planned for a tall and fast icon ride and, as stated, did look at a longer ride too. Eventually they settled for the standard Intamin accelerator top hat-airtime hill combo we have.
Alton Towers were in need of a new thrill ride to stay fresh and competitive, especially after encountering issues with planning for their wooden coaster. With the company in advanced discussions with Intamin over Stealth, they were able to work with them further to get another coaster, Rita. There was likely some sort of deal that they were able to haggle as well. The layout of Rita is kind of just plonked how it could fit really.
It's certainly possible that Rita's layout featured inspiration from Stealth's extended layout, if the design process ever got that far. But it's more likely, in my opinion, just its own separate thing. So history would tell you "Thorpe wanted an Intamin accelerator, which eventually ended up being Stealth. They considered getting a longer one but didn't. Alton Towers also got an Intamin accelerator due to their other plans falling through, and it just so happens that the layout, with some tweaks, would have worked well after Stealth". But to say "Stealth and Rita were planned to be one ride" is a more fun story.
We'll probably never know exactly how things went down (in part because we're never told these things, in part because most who were involved with it have probably moved on), and we'll never see exactly what Stealth's extended layout could have been.
As touched on earlier, Thorpe's MTDP is a very loose thing. It just shows locations and vague scales of what they're considering at the time. But these ideas can and do change (Ghost Train was never mentioned in the 2010 MTDP, for example). The reason Thorpe submitted them is because it made the planning process for the rides easier, and kept up good relations with the council.
When Thorpe do submit plans though, they have to stick to them very precisely. They have little room to change anything, everything must be done pretty much exactly as agreed. So they'd never be able to hide anything. Towers is slightly different, in that a large part of the park is covered by a General Development Order (GDO), which means that, within that area, they can do lots of things without submitting planning applications. This is how they were able to build Octonauts without a planning application, 'hide' some inversions in Smiler's application, and change the profiling on Wickerman's drop.