Don't know about the H-scenes, but I did the obligatory math. One full run of the map each day gets you 32 ornaments/candy canes. If you can clear the map every day for 21 days and collect 672 of each, (seems a lot more doable this event even though at level 49 I'm at nearly the greatest possible commander level gap) you'll require a total of 46,800 jingle bells to finish out both bags at 225K. This works out to 2229 jingle bells per day. If you can't get that many (or can't completely clear the event map) and plan on finishing out both bags, prepare to spend cores in the exchange shop for Holiday Spirit gifts to supplement your event runs. I got 2874 jingle bells just from doing my six invasion tiles, so I'm pretty sure I'll be able to clear the event without spending any cores...unless tile 8 is way freaking harder than tile 7 and I'll end up 84 each of canes/ornaments short (I lost 8 of my tanks on the cane path tile 7, so it'll be a few hours before I can try it out).

If you can finish tile 7 but not tile 8, you'll need a total of 4629 jingle bells per day to make up for the missing 168 items from both paths. Probably not too big of a deal if you have your scouts out for oil.

As for the exchange...according to the wiki, which may not be completely accurate, Katyusha is also available for research (and we already have Minnie, who is a little bit better at the same thing). The wiki says PT-76 Eliza is only available in events (and in my opinion looks hotter). If you're of a mind to spend 2K cores to buy just one of the exchange tanks, my thoughts are to go for PT-76. There's a total of 180 cores in the bags, so if like me you're a bit short to purchase one, that and some invasion tiles could help make up the difference.