nana,
your stray kitty is adorable. i wouldn't be able to resist feeding her either.. i'm not sure what advice to give you..
i agree with june about the litter boxes.. somehow you've got to find ways to encourage them to make your garden and porch their home base. i've got a stray cat that i feed too. i used to feed her under my car, but now i've given her her own food and water bowls and litterbox and placed them right under a window looking out into my garden.. so now she is almost always in my garden and she does her business in her own private attached bathroom
.. she always comes homes to do her business there in peace
when she wants a food refill or attention, she will sit on the window sill and meow loudly into my house.. perhaps you could try the same?
oh and i've spayed her, to discourage roaming around..