Seastead Weather & Navigation Analysis

1. The 1 MPH Reality Check

At a traveling speed of 1 MPH (approx. 0.86 knots), your seastead is technically "underway," but functionally, you are drifting with control. In maritime terms, you are a "vessel restricted in her ability to maneuver."

The "Net Speed" Problem: The North Equatorial Current and Caribbean Current generally flow West/North-West at 0.5 to 1.5 knots.

Heading West: You might make 2-2.5 MPH ground speed.
Heading East: You may actually move backward despite your propellers.

2. Storm Avoidance Logic

You asked if 75 miles of displacement in 3 days can avoid a storm. In the Caribbean, the answer is: Usually not by distance, but potentially by positioning.

Storm Scale: Tropical depressions and large "Easterly Waves" are often 300+ miles wide. Moving 75 miles will not take you out of the rain or wind field.
Strategic Positioning: Your 1 MPH capability is best used to get into the "Lee" (the protected side) of an island. Moving 75 miles to get behind the high mountains of St. Kitts, Guadeloupe, or Dominica is a viable survival strategy.
The "Dangerous Semicircle": If a storm is passing north of you, you want to be on the navigable side (the southern side in the Northern Hemisphere). 1 MPH is just enough to nudge your vessel into a better quadrant if you start early.

3. Structural & Wave Concerns

Your design (a small semi-submersible) has a major advantage: Transparency to waves. Because the water passes through the columns rather than hitting a flat hull, your "heave" (vertical movement) is minimized.

The "Tension Leg" Risk: You mentioned cables connecting the feet. In 15-foot seas, the distance between your floats will experience massive fluctuating forces. If one float is in a crest and the other in a trough, the tension on those cables will be measured in tons. Steel cables can snap or, worse, rip out the mounting points on your columns.

Wave Period: You are correct that "swells" (long period) are fine. However, "wind waves" (short period) in the Caribbean can be 8–10 feet and very steep. These will slam against the underside of your 40x16 living area if it is not high enough above the waterline (Air Gap).
The Air Gap: Ensure your "wet deck" (the bottom of the living area) is at least 10–12 feet above the calm waterline to avoid "slamming" in 15-foot seas.

4. Caribbean Specific Insights

The "Island Effect"

While the Caribbean is generally beautiful, the Anegada Passage and the channels between the islands (like the Dominica Channel) act as funnels. The wind and waves can double in intensity due to the Venturi effect. 1 MPH may not be enough to cross these channels safely if the "Trades" are blowing hard (the "Christmas Winds").

Squalls

Outside of hurricane season, your biggest threat is the "White Squall." These are localized, intense rain/wind bursts. They can go from 10 knots to 40+ knots in minutes. Because you are solar-powered, a heavy squall will kill your power generation exactly when you need the motors most to maintain heading.

5. Recommendations

Redundant Power: If you rely on solar for propulsion, have a backup diesel generator or a massive battery buffer. A storm will take away your sun right when you need to fight the wind.
Dynamic Mooring: Instead of relying on cables between the floats to "hold them in place," consider if the structure can be made rigid with steel trusses. Cables that go slack and then "snap" tight are the #1 cause of structural failure at sea.
AIS and Radar: At 1 MPH, you are a sitting duck for cargo ships. You must have an Active AIS transponder so big ships see you on their screens 10 miles away.
Drogue/Sea Anchor: Since you cannot "outrun" weather at 1 MPH, your best bet is a massive sea anchor. When the wind hits 40 MPH, you deploy it to keep your columns oriented correctly to the waves and slow your drift.