Can phosphorus lead to increased metal staining

[waterbear]

Phosphorous comes in different forms. Orthophosphates are algae food. As far as I know they do not sequester metals. Metal sequesterants can form orthophosphates when they break down.
If you pool has no nitrates, high orthophosphates, and algae blooms that you cannot control then a phosphate remover might be benificial by removing the algae food souce. If nitrates/nitrites are present then it won't since they are algae food also.

Metal Free uses EDTA as the chelating agent. IMHO, it is not as effective as phosphonic acid based seqeusterants. I have used both kinds and I don't feel the EDTA works as well (I tend to keep going back to Proteam Metal Magic...HEDP...a phosphonic acid derivative)
Sequestering agents have a 'chelation index' for different metals. This means that it will sequester one metal more than another...for example, Jack's Magic blue stuff has a high chelation index for copper so it will sequester copper before other metals...so called calcium hardness reducers have a high chelation index for calcium (yes, it's a metal) so they sequester that first. If I am not mistaken the chelation index for different metals with EDTA becomes very pH dependant.....That is why it is used at the titrant in the calcium harndess test at a pH of 12 and used in the total hardness test at a pH of 10 where it chelates both calcium and magnesium.
I have orthophosphate readings well above 1000ppm (I estimate beteen 2000-3000ppm), no nitrate/nitrite and I have NEVER had an algae bloom (nor any CC for that matter!) I do have a SWG, run my cya at 70 ppm and keep my FC between 3-5 ppm.

I think you are looking for problems when there aren't any. Do you have repeated uncontollable algae blooms? If not don't loose any sleep over it.