Here are my thoughts as a blitz main since season 3, and always around master/chall elo since season 4 on EUW.
The passive Mana Barier changes seem overall like a fine idea, this is generally a slight buff for the most part and only would be a nerf if you stack mana items, which apart from getting zekes and righteous glory doesnt happen as a support, there are some blitz players for example Prim0 on euw that play him on toplane and build stuff like manamune iceborn gauntlet, and for building him as a toplane bruiser this would be a significant nerf to that style since your overall mana pool is much higher later on into the game and a 30% shield would almost always be less then the current itteration of 50% of current mana for that style. but generally speaking i think this change is fine overall given the thought process behind it.
As for the static field passive changes, i think its silly to think it causes frustration because for the laning phase there is obviously a clear range for when it does and doesnt proc on minions so that just takes a small learning curve, you can even position yourself in a way so you know which minion it will proc on with just staying in range of the minion u want to proc on and out of range of the other minions, i do think if it would prioritize the closest champion would take away that RNG feeling of the static field in a teamfight since you would know how it would procc and could more so play around having that knowledge, i think would the current path your taking it will dumbify the champion since the frustration aspect comes from lack of understanding of the mechanic of itself and it would also just be a flat out nerf in terms of damage since a good portion of your damage comes currently from your passive in the midgame with just being able to have the reliable dps of it when there is a fight in the river for the drake for example, but the changes your looking to do it would basically take away the mindfullness of applying your cc since you are a melee champion and getting consistently in range to get auto's of to be able to proc it would cause you to potentionally not be in range for the key targets u want to cc with ur E. Furthermore the AP ratio just doesnt make sense since he just isnt played as a mage at all and would take a lot more for people realistically to buy AP items on him. I truely think you could make the static field passive less frustrating if you just would take the current itteration of the passive and focus on prioritization of the ability.
Some personal thoughts to what i think that could benefit blitzcrank are:
Changing the mana cost of his hook to 75 at all levels, in comparison to other champions with hooks it just feels really bad starting the game with 263 mana and a hook costing 100 at all levels, a different itteration of this change would be to keep it 100 and refund 50% of the mana if it actually hits a target, in that way your atleast rewarding being skillfull at the champion and giving the enemies room to capitalize if he does fail it.
His W also costs 75 at all levels with just feels quite high if ur trying to go for a W into Q it costs currently 175 mana, and in the early game that is litterally about half your mana pool. The numbers overall just feel outdated in comparison to other champions that go for their hook combo it is far less punishing in terms of their mana pool and mana costs. i think making his W cost 50 and scale it to 75 at lvl 5 would be fine.
Also his current mana pool seems quite low for a champion that has a passive that relies on his mana for it to be usefull and all his spells apart from his E have really high mana costs.
I hope you look into some of these options and more so will look into giving blitzcrank some quality of life changes rather then completely changing certain mechanics.