Yes, it can be replaced at home. There are two converters - a front unit that is part of the exhaust manifold and a rear unit that is in the exhaust pipe. Both are bolt up parts. The front end would just need to jacked up or driven up on a couple of car ramps. Unbolt the exhaust manifold at the engine and unbolt at exhaust pipe to remove the front converter. The rear converter is then unbolted from the rest of the exhaust system and new ones bolted up. You have to make sure to buy direct fit converters. The less expensive universal converters require welding
Your home mechanic needs to unbolt and drop the converters to look inside each one to determine which one is clogged, so that you are only replacing the bad one.
The constant filling of the radiator will be another issue. It needs to be pressure tested for external or internal leaks using a cooling system pressure tester(this can be rented from Autozone
I wish I didnt have to tell you this, but yes you have two problems, and by your description of steam coming out of the tailpipes, your second problem is a blown headgasket, which causes the coolant to leak internally and get into the combustion chamber and go out the exhaust. It can also mix with the engine oil. Alot of times, but not every instance, the oil will look like milk chocolate.
I would advise haivng your home mechanic replace the head gasket first, then reset the codes and see if the one for the converter comes back. If it does, then the converter would need to be replaced.
But between the two, the headgasket has to be replaced first.
Below are directions you can print up and provide your mechanic on removing the cylinder head to replace head gasket:
Fig. Removing the front suspension member-Altima
Fig. Cylinder head bolt removal sequence-2.5L engine-Altima and Sentra
Fig. Cylinder head bolt tightening sequence-2.5L engine-Altima and Sentra
Before giving up on it, shop around at some independent small shops - you should be able to get the head gasket replaced for around $800.
I realize that is still alot of money but is much less than he told you it would cost...