So I'll soon need to get from HEL over to Lille, which has an interesting location right at the TGV/Eurostar/Thalys train junction between London, Paris, and Brussels. If my Googling is right, it's 1:40 to London, 1 hour to Paris, and 40 minutes to Brussels by train. CDG is right on the train line, BRU to city center is 25 min by train, and going to any of London's airports would take much longer. Flight times to any of the three are pretty much the same. If you value cost over time, which one to take? My gut feeling would be that BRU wins, not least because that way I can avoid CDG...

Also, can somebody explain why attempting to book a journey on tgv.com from CDG AIRPORT (95) to LILLE EUROPE (59) gets me an error saying "Journeys requiring over three changes cannot be booked online"?

I would choose BRU 100%, even though CDG-Lille is direct. Fares are cheaper to Lille from BRU than CDG anyway - no need to book in advance, seats are always available. And BRU is not as crazy as CDG.

At BRU you can purchase your ticket to Lille. There are frequent rail services from the airport to Bruxelles-Midi/Zuid (at least every 20 mins). From here both TGV and Eurostar trains depart to Lille Europe at least every hour with a journey time of 38 mins. Bear in mind that for Eurostar you do have to check-in and go through security and passport control so allow an extra 30 mins.

I think the error you are getting on tgv.com might be because there are two stations in Lille. Try Lille Flandres - I think that's the station which is served from CDG.