We went on the weekdays and had no problem going there

Yes it is. Gets crowded and parking in Lanikai is only available on weekends due to construction during the week.

All water around Hawaii is always swimable and warm

Both routes are quite scenic depending on what you want to see. 72 will take you around the outside with views of ocean, beaches and small islands on the eastern and southeastern shores while also catching glimpses of mountain ranges and highlands to the west with scenic overlooks and stops along the way. 61 will take you through the mountains and more forest areas with glimpses of waterfalls down rock walls and also offers lookouts and stops.

We did see people kayaking right off Lanikai, they prebooked a kayak tour and their guide had their kayaks ready to go on Lanikai Beach. You may have to do some research online.

Didn’t really encounter shells just soft white sand. Absolutely beautiful but not for shelling.

No beach chair/umbrella rentals in Lanikai.

Yes. Parking for cars is difficult but there. Bikes can be locked against the poles at the end of each beach access area. The earlier the better but it’s available. Enjoy!

It will not be any problem with that. You can go easy on yourself

We have been there at around 10am and at 2pm on another day during the week and I would say there have been a few more people later but it made no big difference and you will enjoy the beach at every hour :-)