No, these AmeriGlo GL226 Ghost Ring Tritium Night Sights are designed to fit Glock model pistols. Ameriglo does not make a ghost ring sight set for the Walther PDP. You can find the compatible sight sets for your PDP by using the link below:
https://www.opticsplanet.com/s/walther-pdp?_iv_categories=tritium-fiber-optic-night-sights
You would need to use the AmeriGlo (GL5225) Ghost Ring Night Sight Glock 17/19/19x/26 Gen5, found by using the link below:
https://www.opticsplanet.com/ameriglo-gl5225-ghost-ring-night-sight-glock-17-19-19x-26-gen5-green-tritium-w-o.html
No they won't.
No, these are for the large frame 45's.
No, these will not fit a Mossberg 590. We do have a couple front sight options that might though: https://www.opticsplanet.com/meprolight-tru-dot-shotgun-night-sights-for-mossberg-shotguns.html. The first option fits if you currently have a front bead sight, while the second option should fit if you have the rear ghost ring sight and dovetail front sight.