1. Which kind of magazine feed is better, from the top (like the Bren) or from the bottom (like the BAR)?
The Bren was a superior weapon to the BAR in the role it was intended (squad-level light machine gun). Larger magazine capacity, more comfortable to carry, and its top-feeding magazines were easier to reload than BAR magazines. It's important to note that the BAR was designed as an automatic rifle (and not as a squad LMG) so its intended role was different - also important to note that the concept of a magazine-fed LMG was relatively new at the time too.
Top-loading magazines for LMGs are superior because gravity assists with the feeding of the weapon. Even if the magazine spring is weak, or fouled with dust/dirt/mud/ice/whatever, the rounds will still feed.
The Bren was used actively into the 1980s; the BAR was not.
2. Is feasible to change the bottom feed to a top feed or vice versa?
I mean I suppose you could, but it's a pretty big change and probably wouldn't be worth the time and effort.
3. In a LMG like the Bren in the British army, why it can be attached to a tripod (if the role of med. MG is fulfilled by the well tried Vickers)? How effective can be in the sustained fire role?
I think the tripod was meant for anti-aircraft use.
4. The british army allways insisted in having the same ammo for rifle, LMG and medium MG. Is a reasonable decision. But with an intermediate round do you think that the med. MG must be chambered to this round, or it can still be chambered to a full power cartridge?
Well as far as I can tell, the 762mm FN MAG (L7A2) was used alongside the L85 / SA-80 well after the 7.62mm L1A1 SLR had been retired. I'm not sure what drives these decisions though. A .30-caliber/7.62mm medium machine gun is basically a requirement in my mind, though.
5. The sniper can use a special round, do you prefer a round compatible to the Med. MG or GPMG, or a totally new one?
Historically the round used by NATO marksmen was always the 7.62mm NATO (in modified M14 rifles, Remington 700, H&K PSG-1, G3-SG1, Enfield L42A1, etc.) Only recently have the rifles firing the specialized sniper rounds come about (L115 arctic warfare magnum & US Army M2010 ESR in .300 Win Mag). I think if a military can afford to send snipers into combat with specialized rounds then it should.
6. I know the value of cheapness in a submachine gun, I want to know if it is feasible to improve the well tried wartime submachine guns like the Sten or PPS with refinements (Sten Mk 2 was very crude!) but without making them expensive weapons.
Look no further than the Sterling SMG, Vigneron M2, or MAT-49!