Yes, cloning, repeatedly, maintained, could make you immortal.
There's some logistical concerns that make this trickier than the spell itself:
Vessel must be undisturbed
So, ideally, you set up a nice young version of yourself, hide it away for the time something goes wrong and go about your life, right? Well, the longer it's around, the more likely, over time, something COULD happen to it. Especially in a world where you've got things like purple worms, umber hulks and bulettes and other critters that dig through granite like butter.
Well, then it makes sense to set up some defensive measures, right? Traps, spells, etc. But in the world of D&D, the more defensive measures you put up, the more people assume it's got something valuable to steal...
Now, as a GM I wouldn't just automatically assume something is going to happen, but if the clone is sitting around for decades, or the wizard in question has enemies seeking them out, then we'd start having to think about problems.
A giant diamond
So, the diamond is worth 1,000 gp. This doesn't mean you can simply pull out 1,000 gp and find these diamonds anywhere, everywhere. There's got to be a limited number of them. You're probably not the only caster who is looking into this spell.
So, a bunch of wizards want immortality from a limited resource and are all looking for it.
I'm sure that's not going to lead to problems.
Welcome to the Immortal Club
So, if you manage to live far beyond even what most D&D world folks know people to be capable of, and you're known to be an awesome wizard... how many other people are going to be trying to get your secret of immortality from you?
How do other things which are immortal feel about this? Do they find a way to manipulate/play you because you're new to this game? Do they already have a control on the 1,000 gp diamonds and dole them out to the few wizards who have Clone just to keep them under their leash?
Is there an alliance of lichs who are jealous you've found a way to live, but actually live, not undead live, and they'd like to simply stomp you down for being audacious?
Are there mind flayers looking to eat the juicy mind of a super-intelligent wizard with 800 years of tasty-tasty knowledge?
Are there divine guardians of life and death who did their accounting and finding there's a soul short that needs to move on?
Gameplay
Unless you're playing a very unusual game of D&D, these issues aren't likely to come up too much simply because the timescale is too short. But it makes excellent source of adventures based on NPCs - just imagine what happens when you do have a wizard who has been doing this and dealing with all of these problems and what that means for the PCs when they get involved in it.
No, Warlock Spellslots do not count toward the Multiclass Spellcaster table
Spell Slots. You determine your available spell slots
by adding together all your levels in the bard, cleric,
druid, sorcerer, and wizard classes, half your levels
(rounded down) in the paladin and ranger classes, and
a third of your fighter or rogue levels (rounded down)
if you have the Eldritch Knight or the Arcane Trickster
feature. Use this total to determine your spell slots by
consulting the Multiclass Spellcaster table. - PHB p. 164
Warlock isn't included in that list. Thus you would only have 2 1st level spells as a level 19 warlock, level 1 wizard PC. You could cast level 1 warlock spells using your wizard slots and cast known level 1 wizard spells using your pact magic. Granted you would be able to cast those level 1 spells at maximum level (as per usual with pact magic) but its not going to benefit you in comparison to Eldritch Master which allows you to double your spells per day effectively.
Best Answer
No, you don't regain expended spell slots
Assuming this clone was created by the Clone spell, the spell description states (emphasis added):
This implies that abilities and related things are tied to the soul, rather than the body—for instance, you still have any new abilities, levels, new total spell slots (which may or may not be available before a long rest), etc. that you've gained since the clone of the body was created. Since the original wouldn't have had the ability to use those expended spell slots, neither does the clone.
A well-regarded answer on a related question espouses this same principle for why a Cloned character does not regain the ability to cast Wish: spell-casting abilities are tied to the soul, not the physical body.