I don't like them either. I think they give far too many excuses.
#1: I'd far rather just say "I don't have any spare money." Otherwise, yes, it DOES sound like you think they're untrustworthy.
#4: What's wrong with, "I really enjoy the job I have, thanks"? Why suggest you might take it some time in future if you won't?
#5: Why not just, "sorry, I can't do it this year; I have many other things on my plate." Why say "if I do this, I'm totally going to end up hating all of you just like every year because you pile so much on me"?
#8: How about "sorry, I need it then" or just "I don't lend my car" or "why don't you rent one?" or "can I give you a ride to the shop to pick it up instead?" or "I think Aunt Jane also mentioned she needs to go grocery shopping; maybe you can go with"? Again, why make it sound like they're untrustworthy or like you put a dollar amount on stuff you'll lend (or next they'll be asking to borrow something of yours that's worth $999 that you also won't want to lend)?
#9: Seems to me the polite thing to say is simply, "Oh no, you're a guest, of course you don't need to bring anything." Duh. It makes you sound nice and avoids awkward excuses or leaving the person an opening to argue about how your menu could probably take just ONE MORE THING or feel like her dip isn't good enough for your menu.
#10. This one i have no clue about! Yet another reason getting married would be a fiasco for me--there's no way I'd want a shower (if everyone is already giving a wedding gift, why on earth would I want to ask them for ANOTHER one? Plus I'm not big into showers, period). But there's really no way to say no without someone who's a traditionalist and thinks a wedding isn't proper without elements X, Y, and Z thinking you're just trying to be a jerk...