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I did almost all the same except that for 1) I forced the seller to send me a replacement. The stock diamond hotend didn't print becase it had a 1.4mm diameter for the filament instead of 2.0mm as per RepRap's Diamond Hotend drawing. Even their push rod wouldn't go in the stock hotend for me.

I haven't yet tried lubricating the filament/tubes. Will give it a try. Anyway 15mm/sec speed is waaaay below the expectations for a delta printer that are said to be known for high speeds. Besides, color mixing is actually impossible, so I'm thinking about giving up on this printer and selling it or salvaging it for parts.