Initial Form 5

[WhaleDriver]

Is it possible to complete an initial Form 5 on a Seaplane add on checkride with a DPE?

[BJD]

Even if you could, CAPR 70-1, 7.1.4 says: To operate as PIC in a CAP airplane, a member must pass an initial CAPF 5 in each CAP
aircraft type (e.g., make/model, endorsement, etc.) that they desire to fly.

Beyond that, how do you demonstrate required Soft Field and Short Field Take Offs and Landings in a Seaplane?   

[PHall]

Even if you could, CAPR 70-1, 7.1.4 says: To operate as PIC in a CAP airplane, a member must pass an initial CAPF 5 in each CAP
aircraft type (e.g., make/model, endorsement, etc.) that they desire to fly.

Beyond that, how do you demonstrate required Soft Field and Short Field Take Offs and Landings in a Seaplane?

You demonstrate your good judgement by not trying to do them in a seaplane.  And judgement is something that is evaluated on a Form 5 Checkride.

[WhaleDriver]

Shorts snd softs are not in the ACS for an ASES rating.