I put a spin-on adapter on my '68 Sprite and I'm VERY glad that I did!
The only problem I had was dealing with the oil cooler. The adapter hoses were not designed to accommodate the oil cooler connection. My fix was to use some iron pipes and fittings from the local home improvement store's plumbing department, to fashion an adequate coupler. This was quite simple and not a real problem, I just wish that the adapter already accounted for this.
The filter I use is a K&N HP-2009. It's got a flow check valve, AND it's the one recommended by the manufacturer of the adapter.
As far as changing the oil without removing the drain plug...sure, I do that on my boat all the time. Of course you'll need an oil pump, and the fill tube on the engine must be threaded to accept the hose from the pump! I've never heard of draining oil from a CAR this way since you can actually access the bottom of a car's engine, but what the heck! - - - It sounds like malarkey to me - -
On their death bed, nobody ever said, "Gee I wish I'd spent more time at work!"
'68 Sprite 1275
'76 Midget 1500