As a general advice, avoid stacking unless more ports are needed in the same IDF. Definitely I wouldn't span stacked switches across multiple floors or buildings. Let L3 do it's job, the smaller your L2 domains are, the better.
Why not keeping both? :p
7000-series should be better particularly on the software side, and pretty much mandatory if you want to be seriously in business with IPv6 (IIRC, no SLAAC on the 6450 and 6610... or some other things IPv6-related).
The easiest way that I can think of is to use a linux box running Avahi with an interface on every VLAN where you want to listen and repeat the mDNS queries to. Be careful with the firewall rules on this machine, or it can be used to "jump" between networks.
I've used two Sunon MagLev KDE1204PKVX with two small capacitors between +12V and GND to smooth out the PWM noise... it's not silent by any means but it's bearable when you sit beside it and basically silent when closed inside a rack.
@dontwanna The serial cable is... well, serial :) You'd have to connect it to a serial port of you computer, not on the network (that's the management interface, a NIC essentially bound to the switch CPU); if you have a modern computer that doesn't have a serial port you can use a USB-DB9...
Can you specify the ONT model number?
The ones I've seen are basically an OpenWRT-stick packaged inside a SFP module, so as long as it's plugged in it has an interface up; the management interface runs on the native VLAN and the broadband connection is available on one (or more) tagged VLAN...