Time to collect some dust...

Grooving is the one job where the fence pickup doesn't work Pablo. I was considering, like you, to have pickups above and below the table. One company, possibly Incra, has started making their plate insert rings like spoke wheels and this design is supposed to help with under the table dust collection.

John is correct about reducing a 4" line down to 2 1/2" or less. There won't be enough air flow for the DC to work properly. If I were using my DC for the table I would open up another port farther down to keep the air flowing at the right volume and speed.

Those extra 4 holes around the bit opening should work the same way Pablo. I don't do much grooving on my router table so it has never been a high enough priority for me to have to deal with it yet.

Tom the baffle you show will cause some back pressure and a decrease in air flow efficiency. How much is hard to say without testing the design with baffle vs no baffle. I think I personally would be more inclined to try and get the flow to spiral around the canister like it does in a cyclone. Hopefully most of what's left in the air stream would settle out before the flow moves to the center and goes up.