First off, keep looking around and talk to as many military pilots as you can. Look up forums that specialize in military pilot training, there are several, there you will find lots of great gouge and very eager new pilots who would be happy to share their experiences.
Secondly, the gouge I'm going to give you is from a NAVY guy who's been in for 15 years and was a training command instructor from 2005-2011, so I have some insight but have a little "higher-up" perspective than a new ensign or 2nd Lt, so take that into account....all that being said, here's what I can offer you...
1. What is the process to become a military pilot? Go talk to a OFFICER PILOT recruiter. Do not waste your time with a general recruiter or a guy who is not in aviation. There are MANY things involved, Medical exam, aviation aptitude testing, physical fitness testing, must be a college grad just to name a few
2. Do certs help get slot? That I really have no idea about...I don't think so, but I could be wrong...I WILL tell you that the guys that had LOTS of prior flight time, SOME had a TOUGH time in OCS and flight school based on them not accepting military training and trying to get by on what they already knew. Military flight training is extremely dynamic and fast paced, and only maybe a quarter of what you knew before will translate to military flying.
3. Joining to be a better pilot to only be a better pilot? The military will make you a better pilot without a doubt. Not that a military pilot is better than his civilian counterpart, but what we will teach you to do during your military training can not be replicated in the outside world. AND we can take a OTS kid with ZERO aviation experience and make him a fighter/attack guy able to take off, fight air combat, deliver ordinance on a target then come back and land on a carrier, at night, in time for Midrats...and he will do that mission 3 years after taking his oath. YOU WILL NOT JUST FLY IN THE MILITARY....flying will be your secondary job after you get your wings, you will have to be a leader of men and women and be able to take responsibility for ensuring their safety, you will spend 10X more hours in front of a computer than you will in the cockpit.
4. Pilot slot when you join? Each service is different in that regard, so do some research on that. I know the Navy/Marines give you a pilot slot, which is yours to lose if you dont pass muster for any number of reasons. I have no clue about the Airforce...when I applied to the Airforce the Major that interviewed me asked what I wanted to do, I said "fly fighters!" she asked if I'd be willing to be a navigator, which I replied that I was joining to fly planes...I'd fly transports too...just wouldn't be a navigator....the Air Force never called me back!
Good luck and I hope this was helpful.
FLY NAVY!