Norwin, I have been following this thread closely since you started it. I think it has been mentioned in a few posts that before you decide on an ebike to purchase, you should consider renting a bike for a day's outing. After reading the many challenges you have identified, I believe this should be your next step. I know there are no ebikes available on your island, but you could answer a lot of your questions if you rented a regular pedal bike for a day.
First, the rental company can help you choose the general frame size/wheel size you need and are comfortable on. Second, you can see how well you do with balance, control, stamina, starts and stops, shifting, etc. by pedaling a few short distances on flat paved terrain. If you do reasonably well with that, it will help you a lot in determining whether you need a bike with a throttle or not. If you find you have problems with balance and control, you might even decide an electric trike is most appropriate. Once you have answers to a few of these basic concerns, it will be much easier to focus your search on the type of ebike that is appropriate for you.
Here is one bike rental I found that looks promising: (Link Removed - No Longer Exists). Even though there is a cost involved in bike rental I really think it would be a smart investment before you lay out money on an expensive ebike. A day's worth of practice on a regular bike will give you a lot of answers you need before you proceed.
In terms of the initial assembly and setup, ongoing maintenance, etc. it sounds like you need some help. Do you have any friends, relatives or acquaintances on the island that regularly ride and maintain a regular bike? If so, maybe you can solicit their help. Most of the maintenance tasks on an ebike are no different than a regular bike, so anyone who is familiar with bike maintenance should be able to do it for you. If you don't know anyone, maybe you could run an ad in your local classifieds searching for assistance.
These are just my thoughts based on the comments you have made. Take them or leave them as you choose.