1. If a single block of Heavy ERA like Kontakt-5 or Relikt is hit, does it become ineffective in a sense that it cannot offer same protection level it had before it was hit?
Or are multiple shots (from APFSDS or HEAT or whatever) necessary to "burn out" a single block?

Heavy ERA blocks are quite large and IMHO if blocks are burnt out after few hits, it would be a problem to tank crews if they are forced to fight consecutive battles without ERA maintenance since protection level would drop after each engagement and drop fast. (just like light ERA equipped tanks, but each light ERA block cover only a small portion of protected surface)

2. Is it possible for high-performance autocannon KE rounds, say, those from 40mm, to "burn out" heavy ERAs fast and lower the protection level of the target tank?

3. Also, I heard that maintenance and replenishing Kontakt-5s are not so easy; (Btn. level machine shop - correct?) does this mean in case of fast-paced war, T-72B(M) and T-80Us would have to carry on with significantly reduced armor protection after initial battles? What's Soviet/Russian solution for this?
Of course tanks that doesn't rely on ERAs would also suffer protection degradation (enduring punishments from all that hits), it would be less severe than Kontakt-5 equipped tanks.

each ERA element will work against HEAT only once. but the K5 shell still provides KE protection, since its pretty thick stuff, and will break the rod up somewhat before hitting the main armour.
so it has full protection vs HEAT only once, but repeated protection vs KE.

i think the large K5 bricks contains multiple ERA elements (side by side, not in layers)

anyways, dispersion of modern tank guns is too large for some blown-out ERA bricks to matter.
spraying the front turret with 40mm is going to blow out the ERA bricks, yes. you're also likely to do other damage, to the main gun, sights and such.

First time hit on ERA container - it would work as designed, as heavy explosive reactive armour. After ERA detonation, I guess it would act some like spaced armour (with HHS screen), some like NERA (remains of ERA cells, with non-explosive layers on the way of penetrator/jet). Multishot performance would be decreased. No idea if is it true for Relikt - it might have higher "resolution", I mean ERA would detonate by cells, not by containers. I think this is the way Nozh is working - round would detonate cell or two, but there would be many left.
In K-5 containers cells are in two layers. Side by side, and in layers (in case of T-72B/T-90 glacis, layers are parallel; in T-80U - angled a bit).
I suppose if a target is static it would be possible to hit twice or more times in one container. But when both target and shoting tank are on the move... it would be many hits needed to "disarm" the most of front ERA containers.

it become ineffective as it will not exist

The back plate should still be in place and be effective like a stand off armour steel plate.

What about Israeli ERAs, Blazer 2 on Croatian Degman and M-84D, and hybrid armor (which one is that? ASPRO-H?) on Sabra and Kazakhstani T-72s?

No any back plate in Kontakt 5.

According to a paper I have, it mentions that the rearward movement of the back plate is actually more relevant in defeating the KEP than the forward flying plates (and that the velocity of the forward flying plates has little effect on the damage done to the KEP). I think it mentions that this is because the back plate will be in contact with the KEP for far longer, since it's moving back and then forward.

Nice picture, The long rod looks like it has a higher L:D Ratio compared to most Soviet Era rounds. Could this be a BM-42M or a western Round?

Question: When ERA plate activates has that penetrator already penetrated that plate or does it activate while it is about to do so? Reason why i ask is if both front and back plate contribute to ERA modules effect or just front plate.

The cover of the ERA box is made of a thick high-strength steel plate; when it is hit by a KE penetrator, a flow of high-velocity fragments is generated, which causes detonation of the ERA tiles. The counteraction of the flying thick cover is enough to drastically reduce the armor penetration of both HEAT warheads and KE penetrators.

http://www.niistali.ru/science/secure_en.htm

both front and back plate contribute, but in case of Kontak-5 there are no backplate on the hull as it is integrated. The relikt has the backplate, you may notice on the derawings the space provided for movement of the back plate.

I don't think it has any relation to the real rounds. The picture is to show the general back plate effectivness in the process of the deffeting f KE rounds.