first to answer your questions..
dive "out-fits" to use on the west side..
i only know of one that has a shop on the west..
and that's ocean concepts at wainae harbor..
http://www.oceanconcepts.com/about_us/oahu_dive_centers
they'll have tanks and anything else you might need to rent..
and OC launches most of their dive charters out of wainae.
which "out-fits" do shore dives on the west side..
both ocean concepts and island divers hawaii
http://www.oahuscubadiving.com/
..they take classes to electric beach aka kahe point
[which is just up the road from koolina]..
but, that's just to teach classes for the most part..
electric beach does meet your "long shallow dive" criteria..
tons'o'fish and turtles and octopus.. et al.
now.. to what all the lurkers really want to tell you..
the season for diving the north shore has just started..
[although typhoon yutu might throw a couple bumps in for this weekend]
for your time period.. the sharks cove and three-tables areas should be diveable.
both are fairly "shallow dives" with "very long bottom-times" quite possible..
with great wild-life and lots of lava-tube swim-thrus..
i had two 70 minute dives there last saturday..
so, if you have trans to the north.. i'd recommend that.
finally.. here's a site covering many of the other shore dives on oahu..
http://www.shorediving.com/Earth/Hawaii/Oahu/index.htm