I came back from two forests, am surprised by this:
but no problem, in a pub, I can argue for hours

so let's look at your maps at
("August 6 - 15") clearly shows a smaller Separatists-held territory than the first one ("July 11 - 15") on that web-page:
and I wonder when the area was minimal in between the two periods (it would have to go through a minimum to be able to grow again)? the answer would be, for example, "August 1 - 10" (that's just what I'm guessing by a cursory look)
And the second question is if you think if it's beneficial for Separatists to extend their territory, I mean at tactical level, instead of going into Urban Combat?
I have been meaning to come back to these questions for the last couple of days.
1) The end of the Southern Cauldron has regained the militia a significant amount of territory and this is mainly safe territory to its rear along the Russian border. It also seems that the militia have expanded up to the North East of Lugansk, again to secure a safe corridor to the Russian border.
This will represent a significant net gain over losses on other fronts.
2) How lost are the losses on other fronts?
I suspect nothing like as lost as Kiev would like us all to believe and the truth is that the only result of much of the Ukrainian offensive is to create territory under no actual control, in reality a large and fluid front line.
3) This leads to the whether the militia really want to expand their territory or not.
I would say that there key objectives is not to hold territory at all costs (otherwise they would not have abandoned the Northern half of the Donetsk region) but to take a secure territory around its strategic hubs and control centres, plus lines of critical communications and to do so with the most efficient front line possible, for the level of manpower and firepower that they have available.
This makes the equation quite variable and will also balance the caution of conservation in the maintenance of a smaller territory against the moral enhancement of expansion and encouraging others in the region and in other regions to join the revolt or start their own.
4) Time is also a critical factor and balances further will the strategy of the possible.
Current militia strategy appears to be to generally trade territory for casualties on all fronts but then in particular to let offensive enemy units charge into empty space on their territory and then close the doors behind them to form cauldrons, where they face annihilation.
This does seem to be the tactic of choice and seems to be generating quite spectacular results. We know that the Ukrainians lost the 24th, 72nd and 79th brigades in the original cauldron. While the new southern cauldron seems to have further cost them the 25th and 30th to date.
More than simply high overall casualties, the loss of whole formations are very demoralising for the pro Kiev forces and a major morale boost for the Pro Russians and the next best morale booster to actually recapturing lost territory.
5) Critically, the situation of 4 is all one way and irrespective of the overall level of militia casualties, they are not losing whole brigades at a time. I think that and its consequences needs no further explanation.
Morale among the militia is therefore very high, despite the desperate conditions caused by the artillery shelling to most militia held towns or cities. In that sense, I think the biggest danger they could face is likely to be over confidence or complacency.
6 No doubt in June/July the militia had to bear the full weight of the Ukrainian army and had no option but to bend in the storm and fall back to positions around a territory that they had the manpower and firepower to defend. Since the withdrawal from Slavyansk the Ukrainians have made remarkably few further gains and failed to hold many of them for any period of time.
August was supposed to be the month that the army finished off the militia and retook all breakaway territory and for the event to be celebrated over next weekend. It is very clearly not going to happen and this is why the claims and accusations from Kiev against Moscow are becoming louder, more shrill and wild.
By the end of August, Ukraine will have thrown everything it can at the rebels and will have fully run out of steam. The militia by contrast seem not only to be able to stand the blow without buckling, but grow stronger and better equipped as a consequence. This will be the time I anticipate the militia will look to start to expand their territory once more.
One final point. The Czech guy you mention is not the first none Russian speaker to be found in the militia. Last week (or week before) there was a big fuss about two Spanish guys found in the Vostock battalion. There was a lot of talk about repaying the Spanish debt for the International brigades of the Spanish Civil War, back in the thirties. You will need to look for links as I did not save any.