1/ 4204<---->2910 connection
You can have the 4204 port and the 2910 port facing it either both untagged in VLAN2 or both tagged in VLAN2. This "trunks" VLAN2 to the 4204. Then, you assign all the ports as untagged in VLAN2.
2/ The best topology is a star topology. Each 2910<---->2610 connection is made of ports which have VLAN3 either tagged at both ends or untagged at both ends. If you don't have the cabling to do this, then you can daisy chain one 2610 from the next. In this case, each of the 2610<---->2610 connections has VLAN3 "trunked" same as the other connections.
As far as QoS goes, I always just configure the default. The way I see it, if you actually *need* QoS, then you haven't provisioned the network properly and it's the design that needs fixing.
3/ You seem to be asking about policy-based routing. Even if the 2910 supports it, which I doubt, this is a complication you should not add to your network. Instead, create the router addresses (default GWs) for VLANs1,2,3,4 on the 2910 with a default route 0.0.0.0 pointing at the Adtran router. For VLAN5, do not create a router address. Instead, trunk VLAN5 to the 5505. create the router address for VLAN5 on the 5505. create on the 5505 a default route 0.0.0.0 pointing to your secondary ISP.
4/ Enable IP routing on the 2910. As above, on the 2910 for each VLAN create a VLAN interface and assign an IP address to it (eg "ip address 10.1.1.1 255.255.255.0"). That IP address is the default gateway for its subnet.