To simplify the feats line, how about
Feats: A guardian juggernaut has the feats of the base creature but does not gain any feats from the base construct's Hit Dice. It does gain any bonus feats possessed by the base construct.
Looks good. Environment, org, alignment look right. Advancement seems to be a special case. Maybe that advanced base constructs and base creatures can be used, but generally there is no advancement while the pilot is "driving" the juggernaut? Do we need a note on how to grant XP to PCs who are made into juggernauts?
Yes, I'm really not sure what to do about the pilot. It would be easier if the pilot became a permanent juggernaut --- just use the LA ---, but that's not how these really work. We're going to have to think about this.
Shouldn't the Str (and Dex for that matter) modifiers depend on the base constructs we write later?
Hmmm. What if we subtract the juggernaut's LA from the CR of the monsters that the juggernaut fights? Then combine that with the other opponents? Seem too fiddly or unfair?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.