Hmmmm....well, I looked carefully at the schematic you attached Gino - and the seal to which you refer is (I am pretty sure...) part number 14, which costs the princely sum of $3.40:
14: OIL SEAL (8X25X8) 91204-MB0-013 1 0 Ships within 2 to 3 business days $3.40
The schematic certainly seems to show that it installs from the OUTside (i.e. the direction which is accessible). If it were installed from the INside (i.e. the direction which would require the engine to be completely disassembled) the assembly lines would originate on the INside of the rear case - as they do for the two parts numbered 19 (locating dowels), for example.
EDITORIAL COMMENT: only an idiot would design an engine with a seal installed from the INside unless there was a crucial reason. While they do not mind creating complex designs, in my experience, Honda hasn't employed too many idiots in recent times and I see no crucial reason for creating such a nasty situation on a cheap little seal which should be fairly easily replaced.
So....if I were you, I would buy about 3 of those seals - juuuussstttt to be sure - and then I would dig out the old one (gently) and pop in a new one.
Be sure to install it in the correct direction, lube it up well and seat it all the way into the pocket. It is a good ideal to take a photo of the old one in-place and then compare the thickness of the old seal to a new one. Then you will have a bit of a guide as to the proper installation direction and you'll know about when it is fully seated into its pocket.
Please keep us posted as you go. This is a challenging job, but you've already done the hardest part in getting access to the darned thing and I'm sure you can get the rest of it done.
Pete