Again, like the brackets you posted about earlier, these can be visually difficult for some children, especially when the sequences are very long and the operations are changing from "+" to "-" by fairly large numbers. It is very easy for them to get confused as to which half of the pair they are working with at any given moment.
In the third example you give:
OU, TX, QZ, SA, XA, UZ, WX ...
it may be less painful to quickly write out the first letters in each pair, i.e.
O T Q S X U W ...
and then use the alphabet from there.
They can then do the second letters from each pair in the same way.
Unfortunately it can take time to write them out, so it really should only be used when children are struggling with this type of question, and they need to be 100% accurate in copying the letters.
My kids did find it reassuring to have this method as a "back up", but what was interesting is that after using it a few times they suddenly found the questions much easier because now they they could really see how they work! After that they were perfectly happy with the standard method.
(I should mention that Patricia will probably storm in to the room at any moment and tick me off for my unconventional ideas on how to do VR!
I defer to her as the expert at all times, but I've been chatting to someone about alternative VR techniques today, so it's a hot topic at the moment!)