Can the Pro One Shower head be removed from the filter unit and replaced with my own?

Yes, it can.

When do you know when the filter needs to be replaced with a new filter. How long do filters last before needing to be changed

They will last up to 9 months. Once the water flow slows way down or you smell the chlorine back it needs to be replaced.

Can I order the filter alone without the shower head? We use a telephone shower so the head is of no use.

I am sorry they only come with the head.

What are the GPM's on this particular model?

2.0 gpm.

does this filter pfas?

No to my knowledge, there is not a shower filter on the market that does. But the good news is it is safe to shower and bathe in PFAS-contaminated water. Neither routine showering nor bathing is a significant
source of exposure. Studies have shown very limited absorption of PFAS through the skin.

Does this come in matt finish?

Sorry no, it does not.

Is there a way i can know the average percent% of the specific chemical reduction of heavy metals, lead, pesticides, chloramines, chlorine, herbicides and my main concern (flouride) for these possible 9 months of the filtration capacity? Also does the efficiency start to decline on filtering the chemicals overtime before i have to replace the filter? Or does it start to slowly clogup for instance? Id love to know what to expect thanks.

Here are the lab test results that show the percentage filtered out.  Test results for Propur shower filter

how much % of chlorine does this filter out, and what do the replacement filters cost?

Chlorine 57% chloramines 71%.

Here is the link to the replacement filters.

ProOne Shower Filter Replacement