I have mixed feelings towards your ideas
For starters , it is totally doable to win a race from last place on Yuri Industrial in 3-4 laps (even less). I can make a video proof of that if you want.
Why are you stuck with rockets/other weaps? Don't need them? Shoot 'em away at a safe position. Never had problem with that, even if missile explosion affects me. Gotta know when & where to release. This might require a lil' bit of skill and experience, wouldn't it? Dropping unused pickups, making it free for enemy would be just plain boring + stupid, since you are cutting the tree branch you are standing on with that.
As soon as destructibles come into play, there won't be as much "dumb fires" I guess. We see the general idea already manifesting in the exploding barrels, however they are yet to function in a way it could be used as an advantage to the player.
I always play against AI on hard, and can say, yes they are more focused on taking you down with weaps (even keeping pickups in stock for you, but hey, it's hard AI after all), and blocking your path. But when they crash, I think they just want to recover back on track. Since the cars are big and they are stumbling a bit & speed difference, you have to calculate in that you or others might have a good chance crashing into them. Happens the same when you do crash too... For them, YOU are the one blocking the path.
I don't mean to polish myself, but actually I am bored with the AI as it is right now, since they make no competition, but I can guess there are some others with similar amount of playtime, feeling the same way about that. Recently as I noticed the AI excels mostly on Transport and Skytrack, there they provide a lil' bit more challenge to catch up to, if they get to lead far ahead. If they would start to fight between themselves more, it would just made the players situation even easier.
The timer on powerup, well, yes, that's one thing I'm debating with myself about too. You made good points about that weakening the leading player, but on the other side, sometimes even with it being permanent, I do swallow in some rocket & gattler fire, and that's just against the AI, firing on sight. Imagine this in MP, where others are syncing their assasins & scorpions & EMP's together. The 1 shot perma shield ain't that OP anymore, right?
Keeping the pack together: i'd like to add mines as a tool for that too. Being in a back position, placing them strategically can severely slow down the upcoming leader car as well, as they either run into them, or sliding down from their optimal route, trying to avoid.
Dunno what to think about the speed pads idea tho, but it feels unfair towards the foremost players. Well, right now starting positions aren't fair too. Be it either luck, which plays a major factor in this game, or pure skill, good players will perform good, bad players will perform bad, regardless of the speedpads bonus, that one is true. Might be really a good idea keeping the pack together.
Still, thanks for sharing your toughts with us!