Maldonado brought one million dollars in order to get those Williams/HRT end-2010 tests according to some sources. Williams(aswell as HRT) is heavily struggling for money, so I can understand their decision to make him test over their official test driver easily.
Bottas chances to get a full time seat in 2011 in any team are/were near to zero. He is very experienced in F3ES okay, but jumping GP2/WSR to get to F1 directly would probably be too extreme, especially in this period of limited testing. He is a mid-term bet for Williams, they never plan to put him in the car as soon as there's a free seat. Don't think he has the money for it either. He'd rather focus on finding a drive in GP2/WSR next year and aim for F1 2012/3, which is what he probably does anyway.
About Maldonado replacing Bottas in the tester seat at Williams, doubt that'll ever happen, first of all because Maldonado's sponsors are pushing like hell to put him in F1 next year, guy has a golden opportunity to start an F1 career right now, he has to take it, waiting another year would be the worst thing to do, for both the driver and the sponsors. And in the opportunity that all seats are taken and Maldonado can only be offered an official tester seat, I guess Williams would still keep Bottas as a 4th driver, they'd have no reason to fire him. And Maldonado would probably find a way in 2012, leaving Bottas back in the 3rd driver seat.