Purple question, another one.

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You get to keep all the slots. However, the enhancement effects are scaled down just like an other type of enhancement. On the bright side, since the purples start out a bit higher, the scaling doesn't hurt quite as much.

You can read about IO scaling on Paragonwiki.com.

I'm not 100% sure I understand the question the way you asked it, so I'll just try to clarify the mechanics.

You do not lose set bonuses just because you lose access to a power. The only rule that controls whether you lose access to set bonuses is that you can't be lower than 3 levels underneath the IOs in question.

Purples ignore this rule. You keep your set bonuses for purples no matter what level you are, and whether or not you have access to the powers you slotted them in. Because I know this will probably be contested, here is proof. This is a screen shot showing my Dark/Dark Defender's Attribute Monitor, power and status trays. She has purples in Dark Blast, Night Fall, and Petrifying Gaze. Notice that she has 30% recharge from "Ultimate Improved Recharge Time Bonus". That's the bonus Purples give. If you look at her tray, of the three powers I listed, only Dark Blast is available at level 14, but she still has +30%.

For non-purples, whether you lose a bonus depends on how many of an IO set you have in a power after applying the 3-level rule. For example, if you have 5 level 38 Touch of Deaths in a power plus one level 40, and you exemplar to level 35, you will keep 5 of those Touch of Deaths and lose the 6th for bonus purposes (meaning you'd lose its melee defense bonus).

For global uniques, such as Luck of the Gambler: Recharge, there is only one IO, so only its level matters. Just as with my purple example, you keep the bonus so long as you aren't more than 3 levels lower than the IO, even if you lose access to the power it's slotted in.

The other effect is that the actual enhancement provided by your enhancers is decreased. Broadly speaking, this effect is stronger the further down you exemplar, but it has some hard to explain progressions and exemptions. (Four-way IOs like acc/dam/end/rech are always exempt, for example, because they slip under an enhancement size limit for the effect.) Purples do not ignore this effect. However, if you slot a whole set or the right parts of 5 pieces, you end up with a ton of damage and recharge enhancement which tends to "resist" the reductions. In the situation I took a screen shot of above, my Dark Blast still had 63% damage slotting even though I exemplared from 50 to 14.

For powers you still have after exemplaring, you keep all the slots. The order you add enhancements to slots in a power (or the level you add them at) don't matter at all. The benefit of the enhancements is just decreased (usually).

I hope this helps.

You are incorrect. You do have +damage and other improvements even with normal enhancements. They are not the full enhancement, but they exist.

Follow the link Panzerwaffen provided for full explanation:

No. It doesn't happen with any enhancements at all. The game does not remember when you added slots. As long as you have the power, you have all slots you have added to, no matter what level you added them at. Instead of docking you slots, it scales down your enhancements.

Firstly, this was already answered and the answer was 'Yes'.
Secondly, the second part is wrong. Normal enhancements and purples work EXACTLY THE SAME in this regard. The level at which you got the slots is completely irrelevant.