real

If they remaster them, then they probably will. They already remastered the star wars ones, so i imagine they could in the future if they feel like it.

yes

you could always take a screenshot at 100% completion

Not to be a downer but it is unlikely to have achievements added since it's a pretty old game that wasn't made with Steam in mind. Its source code is probably untouched for more than a decade and I doubt TT Games would update this game - has it ever received an update on Steam? (genuine question, not sarcasm). Despite this, that would be a cool idea, just unlikely to happen tho.

Correct. Without a remaster like the announced HP Collection you can completely forget about that issue.
The only of the early DVD based LEGO games that ever got an update was LEGO Pirates. Because the quality issues here were incredible. Otherwise they just did not care about patching for PC.
For example LEGO Batman II got an HD map update on X360, probably PS§ as well, but never on PC. Things got better when they moved away from DVD to STEAM.
And if they did not care even back then, why should they now?
So please (for your own sake!) don't waste your time and energy hoping and waiting for achievements in 15+ years old games. Simply won't happen. Ever!