'Pre-opening' training is very much a legal risk on the part of the magus sponsoring it, at least in terms of 'someone else could steal my apprentice'. It's comparatively less of a risk, of course, for members of House Bonisagus, since they can invoke their rights if a member of another House claims the child as an apprentice.
I'd also note that the event cited in the sidebar on HoH:TL p.52 does make it very clear that technically the claiming magus has the right of it by the Code, but is primarily intended as a story hook! Claiming someone who is 'obviously' being prepared by another magi is a great way to start a conflict with said magi, and there are all sorts of ways such a thing could be resolved. While the sidebar tends to focus on the Tribunal politics side of the story, the mother of the child could also hire a famous Intangible Assassin to declare Wizard's War, or Certamen specialist for duel-style resolution, and so on. Claiming the grandson of a powerful archmaga as an apprentice when she really wants him is likely to not end well for you, technical rights or not!
I'd also note that the 'several years of intensive education before actually being formally adopted / having the Arts opened' is specifically noted as a common practice of House Tytalus in HoH:S. I would not be surprised if members of other Houses adopted some similar methods, especially if potential apprentices are noticed at an extremely young age. I see a lot of campaigns hand out additional 'apprenticeship' xp in one way or another, often intended to reflect this or simply intensive tutoring for the first few years before being more thoroughly used for lab work.
Tytalan magi usually choose older children than other Houses, and ensure that they are well-schooled in Latin, Artes Liberales, and Athletics (the subjects favored by the Sophists) before apprenticeship begins. This education usually comes from enrollment in a good cathedral school, or else employs private tutors, perhaps even the magus himself, under the disguise of a persona (see below). The child is kept oblivious of the magical destiny ahead of him. After at least four years of this schooling, the student is retrieved from his tutors and informed of his true path. The master then opens the apprentice’s Arts, and begins his Hermetic training.
That said, I'm getting a little away from the core of the issue, which seems to be Practice-xp in Apprenticeship versus Exposure-xp. To me, RAI is pretty clear here: Any season spent doing 'lab work' - which includes Fixing Arcane Connections and Writing/Copying Laboratory Texts - grants only exposure XP. You can have the apprentice practice skills independently instead of doing lab work, but you can't have them do lab work and practice at the same time.
Note, however, that copying a pre-existing summa isn't lab work - a mundane scribe could do so as well - and would thus fit into the Practice rules for Profession (scribe).