1) I assume a frontal partial would be my best option?

2) is it possible to leave hair attached for 5/6 days at a time before removal?

3) I can get mild psoriasis on my scalp at times. Will a piece make this much worse?
Are there particular bases that are easier on sensitive scalps?

4) can u recommend a base, based on the above info and the fact that I sweat a lot?
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1) yes, a front partial is a good place to start.
There are some people who find its actually easier to wear a system (in terms of blending) over the entire top of the head,
But likewise there are many others that wear just a front partial.
Thing is, you can easily progress to a full topper from a partial if you ever wanted to, but its not so easy the other way around.
2) sounds like a very achievable time for an attachment, but it will all depend upon you finding the right glue or tape to suit your body chemistry & lifestyle.
3) I too get mild psoriasis in my temple hair. Not having attachments over 2 weeks long and using Neutrogena medicated shampoo has meant that mine has never become worse as a result of wearing a system.
You can also use a Scalp Protector which you apply to your skin before gluing.
4) I think you'd do well with a fine Swiss Lace base, probably with around a 60% density and a graduated density hairline.
Because you have pale hair, you'll have no issues with visible knots.