Technically, I think the house rule is restricting the elemental effect from being added to any spell. There's absolutely no restrictions in the book about what immuto can be applied to what magia, and while saying effects that modify damage can only be added to spells that deal damage is perfectly reasonable the official rule as written is "Whatever the FM says." I don't disagree with your house rule at all, but I'm not certain that the exclusion was intended. Animate Construct with the Ice immuto is probably how Rasputina made those gamin. It's kind of hard to argue intent, but we might get someone involved in the design process to shed some light on this.
I never considered a pile of scrap doohickeys that you cycle through, then repair during downtime, slowly snowballing until your scrap pile is big enough to be amazing. I'm sure that's not the intended course of action (though, again, discussing intent is hard) but it's the only way to make it usable outside of being handed something via fiat... although the character art for tinker explicitly shows a metal arm. Is there a particular reason they *don't* get one?
On grimoires: I don't think grimoires are common, but I think the story point is that they want to be found. It's kind of implied that dark forces steer these things into the hands of people who will use them. One of the things I really like about this game is the emphasis on Fate, the Fated, and Destiny. In a game like D&D you're literally rooting through a pile of stuff taken from the bodies of things you've killed, a random collection of brickabrack. In TTB, the whole Fate concept seems to help reduce the 'bad taste' left when you give the player something they're interested in pursuing. You don't even have to pretend that random chance has left something in their path. Chance isn't a thing for these people. I do understand how it might feel like it unbalances the starting gear but honestly, if someone decides to follow Pursuit X day one and then shift straight to a caster class it will typically be one of two situations. Either the player earnestly has a specific concept for the character with specific goals or the player is trying to game the system. Player A will easily win me over and get his grimoire. Player B will wait until I feel like giving it to him and I won't feel bad about the delay.
One big message I get from the book is play for the story and the fun. From the very beginning it assumes a level of cooperation and honesty between players and FMs that many established gamers don't necessarily have. It's far too common for a game master and players to have an adversarial relationship, each trying to win out over the other in some way. This game assumes you're all working together for the sake of everyone's enjoyment. The rules aren't written with the precise legalese that other games provide. If players and the FM can get on the same page, I don't think you need it.