Hi Tom,
Congratulations on fostering a grey. They're addictive, aren't they? 
Spraying five times in the house after a few weeks is not that much - I'm guessing that most of the toilet training is going well, as most greys are partly trained, and pick it up quickly, but the odd "accident" is quite normal. Just make sure you have a really good routine for going out every couple of hours at least and immediately after meals etc. We found that greys took a while to be able to "tell" us they needed to go so it was up to us to pre-empt this and give them a good routine. Generally find they enjoy routine anyway as they would have been used to this in their racing days. I would also keep doors closed in bedrooms or other rooms where you don't want them to go, at least for the time being. Have you listed the times of day when this happens? Any common factors?
The odd growl or snap is perfectly normal dog behaviour - this is how they communicate, and does not mean World War 3 is going to break out. Please don't worry about either of them being "dominant" - they will sort out their relationship themselves quite happily and all dogs will occasionally have a little grumble or grump. We have three and one accidentally trod on one of the others on the way out to the car this morning - honestly, the racket! They can be such drama queens.
With the pushing for strokes, ours do this too. Just gently ignore the one you don't want to stroke - although I often end up stroking both. Ours do this too when I'm grooming them - they love it, and will try to come back for more when I'm doing one of the others. I just gently talk to them and carry on with the one I'm doing. Again, please don't get hung up on "dominance" ideas - these have now been completely outdated and don't reflect how dogs actually interact socially.
Lastly, if you are still worried, have a chat with the foster co-ordinator at the greyhound charity. They should be able to offer you lots of help and support and advice.