Maybe I'm not seeing it clearly, but the hoses look to be centered on the risors and the adapters??

If you had 143 stats and were still seeing 180-190 temps, problem isn't with the exhaust hoses. Are the outer hoses about burnt thru also? Maybe not a sign of inner exhaust issue, but just coincidence they gave out 1st?? Both motors, same problem.... T-stats installed correctly, hoses connected correctly? Just pull a couple plugs to see color.

UPDATE:
Pulled the impeller on the starboard motor, got two veins missing. Found one in the impeller housing and will look for the other piece in the morning. Going to catch my backwash of hoses and see if missing piece(s) come out. I think the builder might have cranked it dry, it has new impellers.

(on a side note, replaced my steering pumps-one went bad, idlers-one went bad, fuel/sea pump on starboard went bad. I think the previous owner must have let this boat sit in storage or no maintenance, should have just bought all new accesories for motor. trying to get the kinks out of everything. Sheez this was one trial run with all this gone bad)

Any advice or opinions to the hoses melting with this new info discovered are still welcomed by me.

Sounds like the missing pieces were or are blocking the water to the manifold or riser. You could put air through to find out. Hope you didn't cook the motors.

Going to try your suggestion of air before backwashing hopefully piece missing didn't make it pass the oil cooler and it will blow out. I started both motors when back at the boat ramp and they sounded fine, so maybe I didn't get em too hot. Kinda thinking that the rubber exhaust may have saved my motors by melting and me shutting down.

You might want to check the bolts on the manifolds and risers to see if the heat affected them, could throw a torque wrench to the head bolts while you're at it.

I would look at the feed lines for the manifolds to see if they are crushed or in any way restricting the flow.
Also, when you replaced the t-stats did or is there a white piece of plastic that was in the housing? If there was, could it be possible you installed it incorrectly and water is not getting to the exhaust on that particular side?
Something to think about.

stick your finger inside the inlet of the cooler on hte back of the block. I always find the missing peices there.

But you still had 40psi. Where is your WP gauge connected at, block drains?

Just curious, did you get any water in the boat or on the underside of the hatch from the holes in the exhaust hose.

Will check torque on bolts, check the feed lines and jumper hoses on manifolds. Builder put in stats, but I will check them. The old stats would go in only rightside up or upside down. I'm not missing a piece with the new merc stats? It was blowing a fine mist of water out burn spots. Wp taken on lower portion of block side. Hoping oil cooler caught the part, oil cooler easy to get to.

Thank you for the suggestions, it's keeping me going!

It would have to be blocked after your water pressure gauge pick up to see 40 psi.