You are showing classic case of low freon, the short cycling of the compressor confirms that. All systems leak a bit and many need a top up each season, that is normal. Do not be so afraid to overfill. You may need a whole can or more. Do it by trial and error a bit at a time. At mid 70's you will see at least 35psi on the low side @ 2000 rpm. The accumulator and low side line will sweat and feel very cold. You can add the freon at idle. Pressure in the can is 70 to 90 psi depending on temperature, pressure on the system low side is much lower as you know so freon will flow, no need to race @ 2000 rpm unless you are in a hurry to fill. Run @ 2000 to check pressure.

That foam is oil and dye, normal. It foams because it is saturated with freon which bubbles out at atmospheric pressure. I would not add more oil at this time as your loss has been minimal and you did add some previously. Tweak the suspected union a bit tighter. The site of a leak will accumulate dirt as a small amount of oil comes out with the freon.
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David Hunter