I suggest you make the trips by train instead of by bus. For Bruges to Brussels you do not need to book tickets in advance, leaving from Brussels to Paris I think you need advance tickets.
1) You don't need to if there are seats left on the bus you want (your ticket includes a seat reservation so they don't sell more tickets than the bus has seats), but it is even cheaper if you buy well in advance, see www.eurolines.be.
2) No, for domestic trains you don't get a seat reservation and it makes no difference to the price whether you buy in advance or not, so just turn up at the station and buy your ticket when you want to travel.
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