Aloha
@nippurmagnum, I would advise against a shore entry for this dive. While you may have had great conditions on your last charter, that is not the norm. I live on Hawai'i Island and have done the manta dive many times. About half the times I have done it, we have had swell which is not forgiving to the shoreline at Garden Eel Cove.
But that being said, I know folks that have dove this from shore, the results of which I can separate into two outcomes:
1) there was perfect conditions and no issues
2) either conditions weren't the best at the start, or worsened during the dive, making for a very difficult and stressful exit
Personally I have not dove the manta dive from shore. To me the safety, convenience of diving from a boat, snacks, hot beverages and meeting new people, is worth the money for this dive. If you feel comfortable with the abilities of yourself and your dive buddy, I wish you best of luck and a safe dive. My advice, scout out an entry/exit with backups well before the dive when there is still light out. Descend closer to the rocks, away from the floating thingys with sharp spinny thingys. Of course we all know, and abide by, the rule of diving with at least 2 lights. I want to emphasize that for the exit, if you don't have a working light and are trying to get out, you're kinda screwed.
My last tip from diving from shore, I would pre-write on a slate or water proof paper "Not with your group, we dove from shore." I guarantee a guide (probably one of my friends haha) will come up to you, frantically wondering who you are, if you are one of their divers, or perhaps divers form another boat that got left behind.
One of your objections was having the dive cut short the first time. If you decide to go from boat, talk to the charter ahead of time, let them know your experience level and ask if you can be the first group in/last group out on the manta dive. It's a reasonable request and should give you another 10 minutes or so.
Have fun and dive safely!