Short answer: No.
I can find no evidence of a will cap. Here are two screenshots indicating that Durand's will is well above 150. This is on a Classic difficulty, EW save I found laying around.
Furthermore, I conducted in-combat testing. Megaera (152 will, counting medals, as reported in-mission) had a 82% chance to Mindfray and 52% chance to Mind Control an Ethereal. Durand (181 will, same method) had a 100% chance to Mindfray the same Ethereal, and an 81% chance to Mind Control. Using Psi Inspire to improve Durand's Will to 211 raised the odds of Mind Control to 100%. If Durand (squad leader) walks near Megaera, Lead By Example changes his in-game will and success percentages to be equal to Durand's, even if she was Psi Inspired and he not.
As to the second question regarding Mind Shields, it is of course advantageous for each soldier to carry an individual mind shield - both for defense and offense. However, note that you cannot equip more than one Mind Shield on the same soldier. See my answer here for a list of limited items. https://gaming.stackexchange.com/a/172431/61910
It's possible there was a Will cap in EU (as opposed to EW). Despite 30 playthroughs, I don't seem to have a save handy with multiple 100+ Will (psionic) soldiers. In EW, it happens basically every playthrough.