Answer to 3. : You'll have to destroy all the resonators before adding new ones.
Answer to 1./2./4. : If you're L1, you'll need a lot of bursters to destroy the portal. A XMP Burtster L1 can make 150 points damage when you stand on the resonator.
A L1 resonator health is 1000 points, so for each L1 resonator you'll need 6-7 bursters.
A L2 resonator health is 1500 points, so for each L2 resonator you'll need 10 bursters.
A L3 resonator health is 2000 points, so for each L3 resonator you'll need 13-14 bursters.
source
BUT, if the portal is correctly protected with shields, (4 very rare shields, each one eat 10% of damages), they will reduce damages by 40%... so you'll need A LOT of bursters and time to destroy the complete portal...
And during the attack, owners can restore health of portal... The best solution would be to go there with many friends fully equiped :)
EDIT:
I found an attack simulator that will help you to predict how many bursters you will need to destroy a portal.
No, I do not think there is an easy button to navigate to the nearest portal.
In my experience, the scanner view will load portals within a range of approximately 450 yards 0.25 miles, or 0.4 km.
If you are moving, sometimes the scanner will show a rectangular portal marker to a portal that was recently inside scanner range. The "Navigate to portal" button on the portal view will lock the portal marker on your scanner screen, but you can only have one active nav portal at a time.
Portal keys in your inventory provide a remote portal view, and you can zoom out a little to see the scanner range around the remote portal.
The official intel map is the best tool for finding the nearest portal when nothing is within scanner range. I stick with mobile version on my phone, and always click "go to my location" (compass crosshairs in the top right corner). Save a bookmark to the intel map on your home screen, so it is just as easy to pull up as your scanner.
I also check the intel map on my desktop before heading out to mentally plot a course. The full site has more functionality, but still loads frustratingly slow.
Best Answer
The information below is no longer accurate.
As of June 13, 2017, OPR is available to any and all level 15 agents, referring to this official Tweet by Ingress: https://twitter.com/i/web/status/874371248536596480
Below is the full message: