Plate tectonics, mountain building, and the Flood

No matter how obliquely your planetesimal comes in, it's going to be travelling at 18,000 miles per hour, the orbital velocity of any close-in earth sattelite. After it hits and comes to rest, all that 1/2 mv^2 kinetic energy will turn into either broken rock or heat. Figure it up: it will be a planet-melter. No feasible amount of water will help you dissipate the heat, either.
Run the numbers. Then get back to us on your results.

By George, I think Randy's got it!

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My planetissimal was held together by chemical bonding in much the same way as our Earth is.

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The Earth is chemically bonded together?????? If the Earth got within Jupiter's Roche Limit, by spiralling in or any other way, it would shatter like a dropped light bulb. You're talking nonsense.

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Further I see no mechanism for this object to slowly spiral in for a soft landing as you postulate. The increase in kinetic energy in a low orbit does not account for the sum of the gravitational potential and kinetic energy in high orbit.

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And it makes little difference anyway - whether the amount of energy that has already been calculated gets dissipated in a day or a century, it will still melt the whole crust, likely with plenty of heat to spare.
And as to formation of the elements: not all isotopes, or even all elements, that exist on earth can be made in non-supernova events. The so-called "r-process" nuclides can only form in catastrophic (=supernova) events, where there are simultaneously enormous densities, high temperatures, AND huge fluxes of neutrons. Europium, IIRC, is one element like this - it can't be built by sequential neutron capture, as the intermediate products beta decay too quickly - only by a rapid sequence of captures not availably in such "placid" events as star mergers.

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This forum could be making history!

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I'm not going to hold my breath, though......

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Heat part of the contents of the planetissimal to form metamorphic rocks and fluids for later igneous intrusions.

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Randy has given a kinetic energy number you agreed with - 8 x 10^29 joules - which must be dissipated for your planetesimal to form continents. That's about 2 x 10^29 calories. Let's assume half of that can somehow be dissipated in mechanical breaking of rocks, etc.The earth's crust weighs about 1.4 x 10^26 grams, of rock with a specific heat around .2 - let's assume .25. (Mass of oceans an atmosphere are negligible alongside this.) A starting physics calculation will show that your impact will heat the entire crust by about 2850 degrees C = 5100 degrees F, ignoring phase changes.It ain't gonna work.