I'm sorry but I don't follow your description of your landing gear, it doesn't sound right to me.
Go to the tower / thunder tiger web site and look at the picture of the cessna on floats. Notice that the landing gear ( float mounts ) are both the same and mount on the float the same. There is a difference in the front set when it mounts to the firewall mount, it only has 1 bolt hole in each side where the main gear has 2 bolt holes in each section of the gear mount.
On the firewall mount (for the floats ) the square flat piece you refered to that has 4 holes in it, does it have 4 or 5 holes ? Mine has 5 holes, 2 at the top & 2 at the bottom then 1 in between the 4. The landing gear ( float mounts ) bolt to the bottom 2. Then the top and center 1 line up on the firewall where your original nose gear mounted and will be bolted in at that point.
I hope this helps, I'm not big on the computer so I can't send you any pictures but if we go back and forth enough we might figure out your set up.
AS far as the glue goes there is a special glue for ABS but I'm not sure of the name, it's put out by "BFS". I used epoxy and ca on all mine and it worked fine.
You ask about using Epoxy on the motor mount, did you mean the fire wall , if so yes it will be fine just be sure and score any places you are glueing. Now if that wasn't what you ment I'm not sure where you are using the glue on the motor mount because it ( the motor mount ) bolts to the firewall.
If you are just starting to assemble this plane BE SURE and score any and all places you are glueing, I just use a hobby knife and score cross hatches in all the glue areas.
Now that I have totally confused you I hope you can figure it all out. I check back here every couple days and I'll be looking to see how you make out. ENJOY !!! RED