Can an engine be partly seized (after lack of oil)?

Yes you are an asshole. As for the engine seized is an all or nothing deal. The lower oil pressure can happen when you stop the car, the load of the brakes on the driveline and converter can slow the RPM a little. To the issue at hand, if the noises stopped when you applied the brakes then the problem is potentially not the engine, it is the transmission (and when as the last time you checked the fluid in THAT?). If the oil pressure did drop then engine damage is a given, everything from your main bearings (which are the oil hogs) to your rockers, cams and cylinder walls could have been scarred. the next time you change your oil make sure to check for any particles in it.

From what you told us the engine is not seized, if the engine starts and runs at higher RPMs with limited noise then you can start to reduce things. Engine at speed with transmission in neutral; high RPM, no load on converter, engine or tranny, speed on tranny input shaft no speed on output shaft. At idle tranny in drive; Low RPM, full load on converter, full load on tranny, no speed on tranny input or output shaft. Given that data the main parts effected by the 2 tests are the rod bearings and the converter. The rest of the parts in the engine (main bearings, cam bearings, cams, lifters, cylinder walls and rockers) are more or less unaffected by a change in load but very much affected by RPM.

As I said before putting on the brakes in drive can lower the engine RPM so keep that in mind, your next test may be full load and higher RPM (brakes and gas while in drive) At any point if you don't iek the sounds then shut it off, in the even of a coolant loss or oil loss a quick fluid fill and restart is often a good thing (especially in coolant, get the engine temp down evenly while the parts are moving before the pistons solidify into eggs) but after the engine has been restarted and down to operating temp keeping it running will only either do nothing or cause damage, not fix it. The engine runs now so the engine itself isn't seized, try moving the car: if it won't move then you have your answer, if it will move then get it to a shop, fast. Asking us is something but remember you are our eyes and ears, the best mechanic in the world can only do so much if he is given information by someone who doesn't know a whole lot about vehicles. Get it to a place where a trained pair of eyes and ears can diagnose it.

Basically, when you seize an engine you heat up the motor so much that the piston rings start to heat up and weld the motor shut. You probably really screwed up the motor and bearings in the engine. It will probably continue to drive, but it's going to have a much shorter life span. Those clunking sounds could be damaged parts that are no longer in spec. Or maybe u you got lucky and nothing happened. Just tale this as a lesson to take care of your things.

It sounds like so far so good, but if there's any damage to the bearings that will progress to failure when running, the oil pressure will usually drop as the tolerance gap wears further open and creates a place for the oil to lose resistance to exiting the pressurized ports throughout the block. This process usually happens pretty fast, so I bet you're good.

If you lost oil pressure, you scarred the main and rod bearings, the cam shaft bearings, and probably the camshaft, lifters, and crankshaft. It will start wearing much faster now and at some point, something will fail and your engine will be more kaput and not work at all.

Yes

Read your first sentence. Seized means it is or isn't. There is no partly about it. You also need to learn how to spell BRAKES.