I can only answer your first question. Yes that is possible and in fact Jan (Janix) has 20x10.5 rims wrapped with 315 rubbers all around on his 4.6is without any problems.

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Dinan S1 pkg / 22" Iforged Swift - Toyo 295F*315R / Eisenmann Race - OEM tips / H & R 15mm front
30mm rear spacers / Euro-spec headlights / CF M Competition steering wheel / Full CF interior /
6000K D2S / HID fogs / AiBrights V3 / Interior Xenon / LED sidemarkers / Matte black grille & hood vents