Should I glue down or top nail the first and last row of hardwood engineered flooring?
You have another very viable choice too. There are two parts to this one. 

First you arrange the design of the planned for final moldings to include a combination of baseboard and shoe molding that will cover up the flooring expansion gap and then some. Knowing how much additional overlap there will be of the molding over the top edge of the flooring will help you to know how close to the edge of the first flooring strip you can surface nail and not see these nails once all the trim is in place. Since you say that your expansion gap needs to be 1/2" I would suggest that you may want to plan to have the baseboard installed above/after the flooring. Thus you get the thickness of the baseboard plus the shoe molding to cover the expansion gap plus the first row surface nailing.

The second part of this is to realize that there is the old fashioned way that flooring was installed in the days before pneumatic automatic nailers. You can hand nail the leading edge of the first few rows of flooring using the hidden nail technique applicable to your particular type of flooring. Once you get a few rows down the automatic nailer will be able to be used. 

When I installed a hardwood floor I had a choice to make as to whether to purchase a pneumatic nailer or to hand nail the whole floor. The project was in a fairly narrow room and when I looked at the percentage of flooring runs that had to be hand nailed anyway I ended up not buying the machine and just did the whole thing the old fashioned way. Although I did use the hardened type of flooring nails that have spiral ridges along the length. These nails will provide more resistance to coming loose over the years as the floor goes through seasonal changes. 



I would also tend to suggest it may not be the best idea to glue some of the flooring down to the sub floor. This has two problems. First and foremost is that flooring that is nailed has some flexibility to move when it expands with seasonal changes. If you glue down a part of the flooring you will loose that flexibility and maybe even risk having future buckling in the area of the glued down strips. The second issue is that gluing precludes putting down a material between the flooring and the subfloor such as rosin paper or other material as may be recommended by your flooring manufacturer.