Switch – How to agregate multiple traffic at Layer 2/3 between different vendors devices

Actually if your access switches connected to the so called "core switches" have just one uplink, either to the HP or to the Juniper, full redundancy is not an option given your constraints of no additional cabling.

If also an additional cabling from the WAN router to the HP switch is not an option, even resiliency to the Juniper switch failure is not achievable as far as I can see.

Back to interVLAN routing, any FHRP protocol (VRRP etc.) can effectively work only assuming that a client is able to reach the backup gateway if the active one fails, but in your case if the Juniper switch goes down there is no way for the clients (connected via the switches with a unique uplink to this Juniper) to reach the HP switch. Furthermore, even in the case you have VRRP in the example just above, the clients connected via the HP would only be able to establish interVLAN communications only with clients connected via the HP (no WAN or anything else).

Hope it helps.

Do you have enough router ports available to do this ? You would still have your five trunks between the two buildings, but an outage on SW1 wouldn't bring down the whole LAN anymore.

You would need to finetune loadbalancing.

Ideally you could maybe add a second uplink from switches 1-5 to SW4, as well as from 6-10 to SW2. And maybe an extra portchannel between SW1-SW3 and SW2-SW4 :

Bottom line : with the same capacity between the two buildings, you only have one single point of (total) failure left : the router.