Practice backing in a parking lot until you're OK and confident at it.
You really don't need 4wd or a parking brake to launch a 14 foot aluminum boat w/ a 7.5 on it. Put a piece of wood under the tire if it makes you feel safer. But get the brake fixed.
The wife does not need to help much to launch the boat. Here's some tips:
When you get to the ramp, stop at the prep area and unhook the transom tie downs, put the plug in, tilt the motor up and put all of your stuff in the boat.
Get in line at the launch point, and slowly back down the launch, staying in your lane.
Now, not knowing what kind of trailer you have, I'll assume it is not a full bunk, drive on trailer, but a trailer with rollers in the middle and short bunks at the back.
Back the trailer in until the rear trailer crossmember is just submerged. You should not have to dunk the trailer wheel bearings in the water.
Put the truck in park, throw yer wood under the front tire
and go to the trailer.
Grab the bow rope and give it to your wife standing on the dock, unhook the winch strap or cable, and push the boat off of the trailer.
Have her walk it down the dock and tie it off while you go back to the truck and drive it out. Don't forget the wood!
A full bunk, drive on trailer is a bit different. They get dunked in the water and the boat floats off. Lots of people have someone in the boat and they drive the boat off.
To load the non-drive on trailer, reverse the steps, except pull the winch strap/cable out and hook it to the end of the trailer before backing it down. The back trailer roller should be barely submerged. If you dunk this type of trailer (non-drive on) too far in, the boat will get on it crooked.
Here's where it gets fun:
Have the wife on the dock (you did take her fishing, didn't you?) get the boat moving toward the trailer, and she throws you the bow rope. She will miss you of course, but don't yell at her or she won't go again. Grab the boat, and hook up the winch hook to the boat. Meanwhile the wife has gotten down from the dock and can start to winch the boat up. One it is going up the trailer straight you can go up and finish winching.
Me and my 9-year old can get the boat on the trailer in about 2 minutes, so it sounds more complicated than it really is. The important part in getting the boat onto a non-drive on trailer is to not lose the boat between the dock and the trailer. I can do all of this by myself, but that is a bit more involved.
A drive on trailer is just that: you submerge the trailer and drive the boat on it. They have full sets of bunk to (hopefully) get the boat centered on it properly. Much less involved.