How is it possible to get faint green and disappearing tracks when stripping photoresist?
I recommend you approach the problem in a different way.

You have two different parameters to control for, exposure time, and developer time, so it may take several runs to find out what works well.

Rather than try to make a working PCB, instead try to calibrate your process.
You could be as much as an order of magnitude wrong on the exposure and developing times.

Don't waste an entire piece of PCB on an experiment, use small strips.

You may need to calibrate the exposure time. The process for that is very similar to making a 'dark room test strip', and is described here https://electronics.stackexchange.com/a/254412/9006

Then, you need to refine the development process. Make the developer up exactly as specified by the manufacturer.

IIRC, we 'wash' the developer across the board with a soft brush until it has clearly changed.

Design a small test PCB (say 1cm square), containing tracks and spaces at the spacing you need, and one 'size' narrower, and one 'size' wider. For example, if you are aiming to make 10mil track and 10mil space, make a pattern with 8mil, 10mil and 12mil track and space so that you can see what is necessary for development and etching. Replicate the test PCB pattern across a strip of material, say 1cm by the width of your pristine PCB 'blank'.

Then go through a range of exposure times for the PCB strip using the 'test strip technique'

Do not just put it into the developer for a fixed time and expect it to 'just work'. You might be developing too long, or not long enough. Try to develop, and keep examining what happens, say every 5 seconds, while keeping track of the time. 

The first experiment might fail, but should 'bracket' (i.e. bound on both sides) something which approximately works, then refine by using finer time steps, so that the time for exposure and development is within 10 seconds.

Once you have a process that works, and is repeatable, then it becomes much easier. With a stable, well defined process, you will be able to use finer track and space.

We use 'proper' laser-printable PCB film, but friends say they use tracing paper with good results.

Edit:
We had to futz with the laser printer to get good black and solid tracks with crisp edges. It was an expensive office printer, but its settings were optimised for nice looking photo's, and to make the toner last. So we had to try permutations of printer settings, and examine with a magnifier or microscope. For large areas of black, we found it helped to have a fine hatch (maybe 10-25%); for some reason the areas got much sharper. (We theorised about the charge on the drum leaking, but never got to the bottom of it. Instead we always used fills with a hatch, and they worked fine.)

Also, IIRC the printer claimed higher resolution than we could get. It claimed 2400 dpi, but AFAICT used different droplet sizes. It appeared, on the settings we tried that it got some of its resolution by varying the dot size, and by image processing the image, but couldn't actually put a dot at any of the 2400 positions across an inch. When we looked at fine detail through a microscope the edges of lines were 'fuzzy', though they looked okay to the naked eye. When we developed and etched the board, the 'fuzz' was a low-resistance short circuit. I think we ended up treating it as a 600dpi printer, and I redesigned a few footprints to enhance the space between pads so it was slightly increased. We were doing 10mil track &amp; space.

Summary: making PCBs is a very 'analogue process' and all the steps need to be understood and characterised, and that may need quite a low of experimentation. Once the process is well understood, it can be repeatable, but until then it can be frustrating.