I thought you could use other lanes (at any time) as long as the "T" icon is showing on the overhead gantry? Mind you, then your in with the others queueing for the other forms of payment?!
That's what I thought, the second time I used it. But by the time I'd realised that none of the lanes were showing the 'T' icon, I'd missed my chance at taking the wide loads exit.
I went to a staffed booth, where the nice lady took my tag number and let me through. She told me I should always use the wide loads lane at the main toll plazas in future.
I believe the toll booths on the exits usually have one lane designated for tags only.
To be honest, I probably don't use the toll enough to really justify a tag (average about 3-6 times a month) but I'm sick of trying to dig my wallet and credit card out of my pocket while driving. I always forget until I'm on the toll road, so that means having to dig around for payment while driving. That's always struck me as one of the dangers of toll roads - you're far from having your full concentration on the road while you're trying to sort out the means to pay. Of course you could start looking for your payment after stopping at the barrier, but that won't make you popular with the drivers behind. And of course in the likes of the Mersey tunnels, whether or not you have exact change dictates which booth you should pull up at, so you have to have payment sorted before you stop at the barrier.
The other problem I have with the M6 toll is that I tend to just chuck my wallet and card on the passenger seat once I've paid. My favourite trick then is to remember to collect my wallet, but leave my credit card on the passenger seat of the car. A tag seems to make sense if it'll stop me doing that.
Back to the positioning of the tag. I think you'll be fine as long as the whole of the tag will fit within the dotted area. I doubt that left or right mounting will make much difference. If you need to remove it and move it, then good old 'No More Nails on a roll - Permanent' will probably do a good job of replacing the original adhesive strip.