First off...
Sure the rhemoraz can burrow. Sure it likes to hide under the snow and jump out. Sure, thats its best method of attack.
However when something this large burrows through the ice within 20' of the surface, is it hidden? is it quiet? Does it leave no sign of its passage? Does it automatically choose the right spot to sit and wait so that INTELLIGENT PCS walk right over it so that its not gonna take more than a 20' move to get them?
Well gee, we can decie "yeah cuz i liek ambushing parties it succeeds at all these automatically and nothing the party can have can ever make a difference in this",,,
or...
we can decide to use the skill system. Those things would be spot vs hide, listen vs quiet, wilderness lore vs wilderness lore, etc etc etc. looking at the rhemorazs stats, i doubt it will win these checks vs most any party unless they are a bunch of citified buffoons out in the wilderness tundra with no guide.
Sure, it can probably successfull ambush it normally not-as-bright YAK food source, but most parties, not all, are brighter and more skilled than a YAK.
There is nothing in the description of burrow or tremorsense which says "and this translates to always everytime perfect ambush without and skills to back it up."
Do you have every creature with tremorsense and burrow always have surprise? Umber hulks get real nasty with automatic surprise.
Without surprise, the rhemoraz can easily be avoided. It can be handled by spells or at range. it has a plethora of weaknesses for an inteligent equipped party.
its at best terrain, denying the party an area while it sits in its bunker waiting for its next yak-intellect meal to walk by.
I'll bet a lot of monsters in the book have "bad" crs when auto-ambush is added to their special qualities line by the GM. Especially if he hasn't read the DMG section on adjusting CR for surprise and ambush.
As for the timing thing of the bite/grab/swallow... the DND FAQ has a set of examples for that. IIRC its a 2-3 round process. hit, make grab, hold, then swallow whole.
but check the faq for specifics.