Stupid questions are actually rare. Stupid answers are much more common.
ad a), for the failover at the 3011 it makes no difference whether the second WAN interface of the 3011 gets its IP configuration directly from the LTE modem (using the passthrough method) or whether it gets it from the LTE router, which in turn gets another address from the LTE modem. The only difference is that there is the additional interconnection subnet, but that is not in conflict with anything, it’s the same as if that WAN was connected to an ADSL modem in router mode, except that you don’t need a double NAT as you can set a route to the subnets served by the 3011 at the LTE modem, which is not always possible on ADSL modems.
ad b), since you need wlan1 and ether1 to be bridged together, you have to attach all /interface vlan to the bridge, not to ether1.
So it would look like this:
/interface bridge
add name=bridge1
/interface bridge port
add bridge=bridge1 interface=ether1
(the wireless interfaces will be added dynamically by CAPsMAN)
/interface vlan
add interface=bridge vlan-id=88 name=bridge1.88
/interface lte apn
add add-default-route=no apn=internet name=passthrough use-peer-dns=no passthrough-interface=bridge1.88
/interface lte set [find] apn-profiles=passthrough
I apologize in advance if VLAN interface cannot be used for passthrough, I haven’t tested this setup practically yet.
Regarding the bridge, this post may help clarify some confusion.