How do I watch replays that I have downloaded but are from previous patch?

No, you can watch them until forever.

I have some ideas, yes. First try not to make the recorded video too long. This is always a bad idea as you don't go in-game to watch the recorded video. You watch it in client. Try to open a highlight from the file that it's saved in. You can do this by clicking on the "file" icon that next to the highlight name, next to the trash button, too. This may help. Yes the records are really laggy. You can try to uncheck the "Low spec mode" as this is not recomended it might help the video but it won't be too good for the over all client performence. You can find the option in the option menu in the right top corner of the client, between the "x" button and the "?" button. After that it will be the first thing that you will see. It will be probably checked.

I hope that will help in some way but remember the Highlights are laggy. I have prrety much the same problem. If you need more help feel free to ask and please tell me if this helped.