Ahh, you're doing hardcore AND expert mode? Alone? Heh, yeah, good frickin' luck with that one, padre.
Don't even bother with melee pre-hardmode, it's not worth it. You can't soak enough damage to make getting that close worthwhile, and the longer-ranged melee weapons pre-hardmode are almost universally lackluster (exception: Dark Lance. Also, if you get a basic spear VERY early on it can help push back enemies until you get a real weapon, but killing anything will be a chore and it loses effectiveness after zombies/face monsters). Instead, get a bow ASAP and start pumping out frostburn arrows using ice torches.
At the first available opportunity, pop a crimson heart/shadow orb and then wait for the meteorite to fall. The meteor heads can be a pain to deal with, but getting a full space suit so you can whip out your fully activated spam gun is essential to getting anywhere. If you're in a corrupt world, the musket you get from the orb can almost be useful as a weapon... If you're in a crimson world, the undertaker is completely worthless.
Nearly every trap will kill you if you set it off by accident even once, until you're at 400 health and have decent armor. If you want to Not Die, you need to be exceptionally vigilant at all times, which is a massive pain and also an unreasonable thing to expect of anybody. Luckily, there's now the mechanical lens item that will point out every single trap in the game! Unfortunately, it's only sold by the mechanic, who only shows up after you've defeated Skeletron, and Skeletron is murder on expert mode. Instead, try to get an alchemy station going as soon as possible and stock up on shiverthorn and cobwebs for dangersense potions. They help, a lot. They'll even help point out places of interest like spider dens and whatnot.
Build little rope-accessible watchtowers at regular intervals, especially around dangerous areas such as the crimson or deserts, so that you have a bolt-hole ready for any sort of large spawning event or difficult enemy. DO NOT go anywhere near a desert during a sandstorm, the enemies there can and will butcher you in seconds (and cannot be outrun) and the wind screws with your sense of movement.
Also, regarding the jungle's own abilty to spread... Technically it can do that, yes. In practice, chlorophyte clusters will readily get overpowered by an encroaching corruption/crimson, and chloro's ability to spread is considerably less powerful than that of the properly spreading biomes. And even then, most of the jungle is unprotected by chlorophyte even when it does begin spawning, so a fair whack will get converted anyways.
But again, as stated before, it's predominantly a matter of principle regarding the fact that the world you've grown to know and love has decided to permanently transform itself into something completely different, and there's not much you can do about it that doesn't require drastic measures. The dryad pushing you to "fix it" just rubs it in.