a) They're absolutely compatible with Ting. My wife and I are using two
Lumia 435's without any specific problems at all. Because we're using T-Mobile branded Windows handsets, we get WiFi calling capabilities as well with Ting, which can be handy in hospital basements and whatnot... though MMS over UMA WiFi doesn't work, nor does purely sending/receiving MMS over WiFi only work - you have to use mobile data.
b) The MMS problem would still be an "issue" in that you would still need to have mobile data active at all times, but (and this is a huge but)... the inbuilt Data Sense manager? Best data manager on any smartphone platform. Aggressive, too, if you want it be. I'll give you an example: shared data usage between our two Lumia 435s runs around 50MB a month
total (we each have the phones set to a 50MB "soft cap" on the phone end). I'm currently on day 25 of this cycle, and I've personally only used 2.01MB. My wife goes out a lot more and isn't as aggressive with her data settings as I have, and she's only used 44.06MB, and this has been a
heavy data usage month for her. In comparison, my mother's on a Moto E, and I've optimized, restricted and tweaked to my all-fire optimized best on her phone, and the basic Google background apps easily chew up 100+MB a month, and she's averaging nearly 250MB per month at this point - she used to get by with under 80MB/month on the same phone pre-Lollipop. Microsoft's Data Sense actually respects your data restriction limits. Same with privacy and app restrictions to cameras, microphones, GPS, etc. This said, we don't do a lot of MMS-ing.
We do occasionally have to reboot the phones to get MMS data working again, but we had the same problem with our old Blackberry 9900's as well. I suspect it's something with our local network specifically, and it usually happens after we've been in a 2G GSM service area or off roaming.
Regarding sending/receiving MMS via WiFi data,
Ting doesn't officially support it (almost nobody does - they want that sweet sweet mobile data MMS revenue), not even sure you can on Ting. This said, I know you could on T-Mobile, but it wasn't a universal thing, it depended heavily on the phone and/or app. Textra is the only app I know of that can specifically do this, but I also know that Textra had a hack for T-Mo users that disabled WiFi to send MMS with T-Mo users. Textra isn't available for Windows Phone, in fact, no other SMS/MMS handling apps are available for Windows Phone... so if you go this route? You will lose that... but other than not being able to avoid mobile data charges on MMS messages, the Messenger app is quite slick.
If you go the Windows Phone route and you want WiFi calling support, you'll need a T-Mobile branded handset running WP 8.1. Anything that can or does run WP10 won't support UMA WiFi calling. If you don't care about that, go with a carrier unlocked, refurbished AT&T Lumia 640 and update it to WP10 the instant you get hold of it.
That help?
