I think you pretty much covered it in your question. Granite has a grip, it is extremely hardwearing. it will stain however if you don't seal it. the stains tend to wash/bleach out over time though. my opinion is that it being a natural product the slight variations that will occur between each stone are exactly what makes it nice. Ceramic tiles may be the better option to provide the uniformity you require. there are more and more of these being brought to the market and they are extremely popular. the finishes vary and non slip ones are out there. the great asset is that they can be cleaned very easily. Couple that with the ease of grouting/pointing (you will be able to use a wet grouting mix and clean off as you go) it makes for a very practical solution. they will need to be laid on a concrete screed for best results which is an added cost.

Power wash to remove any debris or growth, perhaps use a concrete stain to colour them, use a separator, such as grass, treated wood or pebbles to create a pattern and re-lay the slabs. Use the edging to create a raised flower bed along the back or sides, and you might think about some adding a few sections of trellis to bring some vertical height to the walls and break-up the walls. An evergreen type of climber would provide colour and fullness both summer and winter. A few shaped pyramid junipers might do that as well, but they do attract mosquitos.

Celine there are loads of porcelain finishes from the palest of white to wood effect and dark grey. I would avoid white as it could show the dirt a bit too much! Here’s some more images of porcelain. I don’t know what the paving is in your pics but possibly porcelain?

Looks like rust staining which can happen with sandstone and other natural stone. Try the website www.thepavingexpert.com which gives lots of advice. Their returns policy does not seem to comply with Distance Selling Regulations. When buying something online and unseen, you have the right to return the item without even giving a reason, as long as it is done within 30 days. These are your statutory rights. This doesn’t apply with underwear, pillows and personalised items but pretty much everything else. Obviously paving is more difficult to return but they must have a system for dealing with this and it must crop up sometimes. I would contact the Citizens Advice for help and further advice. I do hope it is sorted soon.