Sunny54,
I agree with Jackal, you can not risk a closure of the Seward Highway - even if it isn't closed due to a motor vehicle collision, it could be due to traffic. Monday may not be as bad, but in July there has been so much traffic that it has backed up for many miles and turned a 2 hour drive from the Seward/Sterling Hwy Intersection in to a 3 and a half hour drive - Sundays are notorious for this when all of the people in Anchorage come down for the weekend and are heading back. Not surprisingly, this is when many collisions occur. I know that many airlines may have limited or only seasonal service, and although I am not familiar with any of the scheduled out of Anchorage except for AS, many of the flights are very full and it may be hard to find alternate arrangement is you miss those flights.
I assume you have lodging already set up for these dates? If not, secure it now - some of the communities you mentioned are small and have limited options.
For July 15 - Given your three options I personally would eliminate Homer. As was mentioned, some people love it (and it has great scenery) and some people don't. I am pretty indifferent, but I think there is more value in the fjords of Seward, as well as things like Exit Glacier. Seward beats Homer for beauty and scenery on a nice day. I would not drive from Homer to Anchorage in one day, It is a very long slog through mostly two-lane highways filled with motorhomes and people with somewhat questionable driving skills. I think that it could potentially be a negative end to a wonderful trip. If you wish to see Homer there may be some alternate ways of doing it, such as returning the car in Homer and flying back to Anchorage, although that could be cost prohibitive.
I am a bit biased, but the Cooper Landing area is my favorite part of the Kenai Peninsula. You mentioned the possibility of hiking to the Falls or driving down Skilak Lake Rd. If you are coming from Seward, it would be fairly easy to do both of those things. The hike to the falls takes about one hour each way, and the hike is not strenuous at all. I compare it to a Sunday walk - just through bear country. Your time may vary depending on your pace, time stopping, etc. You could spend a few hours at the falls, and then head to Soldotna to stay overnight via Skilak Lake Rd. The road is about 17 miles long but has numerous pullouts for scenery, and also many trails if you have not yet had your fill of hiking. Before planning on going down Skilak, check your rental car contract carefully. Most rental cars prohibit driving down unpaved roads, and some name Skilak Lake Rd. specifically.
For July 10 - Depending on how many stops on the Seward Hwy, you may consider taking a quick trip to Hope. Hope is a tiny little town founded due to the Gold Rush on the Kenai Peninsula. Be aware, it is about 60 miles from Girdwood so it may be too much driving. It's a pretty drive and there isn't a huge amount of things to do - tourist activities usually include kayaking or gold mining but it's a place to go if you have the time.
Both of the bear watching tours you mentioned in Soldotna are pretty reputable, also check out Natron Air as a possibility. As far as transportation from the Hertz office in downtown (I assume it's the one by the Hotel Captain Cook) there is a bus system in Anchorage, but I honestly have no idea how to navigate it. Uber is in Anchorage now, or the bellman at the Captain Cook can probably rustle up an old-fashioned cab.
I hope this input helps. Please feel free to reach out if I can help at all, especially with the Kenai Peninsula area. I would be happy to give any recommendations on services in the area, as there are some notorious places that look good on the Internet that are not in real life. I live in Soldotna so would be happy to meet you both if you come down this way, and there is also a meet up thread for ANC that you can post in for the "big city" Alaska FT'ers.