That's the bearings shot ( and really badly) The shaft should turn by your fingers with power off. Needing a wrench to turn it tells me the bearings are toast certainly. Sometimes you can free them up by spinning the shaft and eventually it gets easier. There will be a lot of scraping and groaning at first and be careful not to damage the impeller. There are two ways to spin it without doing any damage and it depends on the motor deisgn. On a Franklin motor, there's a small dust cap on the back shaft end, pry it off and you'll find the shaft has either a hex end for a wrench or more commonly a slot for a large slot screw driver ( a biggie with a 1/2 inch blade at least). You can turn the shaft safely there. For a motor that has a a complete rear end cover that's removed by two screw bolts ( like an AO Smith), take that cover off. Look down the end of the shaft. See where it's flattened? That accepts either a 1/2 inch or 5/8 open end wrench. The trick is routing the wrench in through the side between the posts supporting the back cover, the wiring, the electrical board and the capacitor all the while making sure you don't dislodge the springs you see there and flipping that wrench around a few dozen times. It's fun but can be done.
Once you free it up enough to turn the impeller by hand, then you can try powering it up. Not until then or you'll just cause more damage. The motor will run a little noisy for an hour or so and settle down a bit quieter but the damage is done and the bearings will eventually give out completely. You may get a day, you may get a year out of it. Don't forget to secure that motor in some fashion before you power it up or you'll see it roll out the door from the torque if the shaft spins.
Sometimes nothing works. The bearings themselves are sealed. There's really no way you can spray in there effectively to help un seize 'em. The heat build up and breaker trip occur because that energy has no place to go. It can't be converted to mechanical energy so it goes to heat.
If you know someone that does rebuilds that'll cut you a really good deal, then go that route otherwise, junk the motor. Typically, around here anyway, doing a bearing set is almost as much as a new motor so it's not worth it. Doing it yourself isn't an attractive option unless you have the bearing puller and the know how to get the bell housing off without ripping apart the electrical card. That back end is the hard part and even I would say "screw it" to most of them since they're usually corroded and you've gotta hope you don't snap any of the thru body bolts.
and ohh yeah.....power off, preferably at the breaker and locked out when you're dealing with this. High volt actually does feel a little different than 110. Wrenches and screwdrivers always seem to find a live circuit. :)
Edit: There's nothing inside those motors to inhibit the shaft spinning other than bearings. The windings won't have exploded getting caught in the magnets.
The only other thing that would stiffen up a shaft spin on an unpowered motor would be an adjustable shaft seal that's been set to it's maximum compression. No common pool pump has used that type of seal for 20 years, (the old AO smith on a bronze Anthony Pools wet end being the last example of this type I've seen) . They're all a fixed compression unit now. No, you couldn't have installed the shaft seal too tight. They bottom out where're they're supposed to by themselves. Actually when you first turn on the motor after installing the shaft seal, they move that fraction into position from your hand tighten when the shaft spins and the impeller tries to catch up.
A done in capacitor would humm and trip a breaker, but you can still turn the shaft when no power is going to the motor if the bearings are fine. Heck, even powered up, if the capacitor is toast and bearings fine, you can spin the shaft a bit using the slot I mentioned earlier in the back end and get it moving enough that the motor starts to spool up itself. Sometimes enough to kick start the motor before a breaker trip. At least getting it going until it's shut down again.
Ahh...I read the last part of your edit. Storage for 15 years in a humid area or near the source of a vapor like chlorine could do that motor in, sure. Might even do that in a dry area too. Dunno since you usually never see a date code on a pool motor older than 10 years since manufacture. They never last much longer than that even if you're lucky. I wouldn't be calling that motor brand new either. I'd call it " not used". Heck, my Mom's Olds 88 only has 50,000 km (about 14,000 miles) on it but it's over 12 years old now and has just about every major item repaired from just rotting as it sat in the garage.
Two questions. 1/ Does the impeller install by means of set screws on a collar that slips over the end of the motor shaft? 2/ Who is it made by, by the way? Both motor and wet end.
If the answer to question 1 is no then I can solve this I suppose by asking if you remember if you had to hold the shaft still when you installed the impeller. If you didn't, it would have to be the bearings. That shaft would have spun as you twisted on the impeller since it's not the "really old" shaft seal design that's manually compressed and adjusted but rather the common threaded shaft end that's idiot proof.
Edit again: Lol, that's an old puppy. Haven't seen one like that in years. Glad you figured it was a set screw manual adjust shaft seal and hope the tension is right now. It doesn't require much pressure, just contact and a bit. Man, you had to have bottomed it out for it not to turn!!!
Jet issue could be too many factors to list. It's plumbing dependant. It may be that your booster pump is plumbed in a way that it's drawing from the pool lines via a leaky valve or even a check valve with debris in it. I can't tell from here, you just have to trace the plumbing runs and see where a potential issue could arise. Just remember, water is lazy and takes the route of least resistance. That includes length of run and head. As for cleaners, I've not run across those ones. I have experience in Barracuda, Hayward, Polaris and Kreepy. Most common in my area. Suction based preference would be Kreepy. Booster pump of course would be Polaris. Choice between the two? Toss up. Polaris is more expensive but can deliver a bit more punch on some shaped pools. Kreepy is cheaper, yet delivers a big bang for the buck if you don't mind reduced filtration at times you're not looking after the pool.
Next time, start your trouble shooting from the base and work out. Motor, motor load, motor load wet end and motor load wet end output. :)