Is it possible to "slim" down the lapels of a canvassed jacket? If so, how hard and at what price?

I would not try

The "it's not a big deal to remove pleats" isn't actually true as well. You have to completely disassemble the pants, completely recut the legs, and the reassemble.

You can narrow lapels, but not on patterned suits as the line of the pattern is generally cut to follow the line of the lapel edge. Pinstripes are notoriously bad. Better to just buy a suit in the style you want.

First, it's not that easy removing pleats and requires a pretty skilled tailor as you are moving pockets and recutting. Most common mistake is the trousers come out too tight in the seat.

Narrowing lapels can be done but most cheat and cut the edge and disregard the grain line of the cloth so it looks altered and askew. If the lapel buttonhole is cut thru, reducing at the edge is not possible. There are three ways to narrow the lapel without opening the outside edge but they have to know how and then do it skillfully.

Personally, I don't think restyling of suits is a good idea. Buy the style you want. If the alterations are not done well and you are not happy with the result, you lose in every way.

I would suggest forget about the alterlation for older suits. Spend the money on something new and does not require alterlation.

I wouldn't try. I would imagine that (1) it would be noticeable even with a good tailor and (2) the proportions might be off on the lapel-to-suit after doing it.

Most suits need some form of alteration, especially if you're purchasing an off-the-peg suit. You're very fornuate if you can get such suits to fit correctly and a keen eye can always spot one. If you're buying a suit for 500 you'd probably spend another 100 having it fitted correctly and there's not much point in doing that IMO because you can have a full-tailored suit for not much more.