Hi P D
Answering your question inline:
- Can I "stack" them both together in some way so I could provide redundancy to the dualported host cards?
Yes - using MLAG - see HowTo Configure MLAG on Mellanox Switches
- Can I configure them in ethernet mode without the extra license? So I could connect QSFP+ to SFP+ breakout cables towards hosts, with ConnectX cards and other brands like Intel X520?
No - You need the UPGR-GW-6012 license to be able to change the switch profile to VPI or Ethernet modes
- Can I uplink them with the SFP+ ports of other brand switches for aggregation with an existing ethernet network?
Yes
- Do they support some form of LACP when running in ethernet mode?
Yes