This varies a lot, but it usually takes between 45-90 minutes to get from parks to resort, depending on how long you have to wait for the bus, how many stops there are before you get off, etc.
The reason you book fastpasses for the more popular rides is because there is typically not same-day fastpass availability for them in the app. 7DMT, Little Mermaid, and Teacups are all close together, but the latter two are a "waste" of a fastpass in advance because you can easily get a FP for them once you've used your first three. If you don't get a fastpass for 7DMT however, realistically your only options are either to rope drop it, skip the ride or wait 100+ minutes any time after rope drop.
As for the concern about criss crossing the park, I wouldn't worry too much as MK is not actually that huge. And the way we deal with that is sequence our fastpasses based on a walking path and then ride standby on low wait rides in between. So for example, we might rope drop space mountain, then ride Buzz and astro orbiter via standby, then head up to fantasyland for a 9:30-10:30am fastpass at 7DMT, do low-wait fantasyland stuff (teacups, mermaid, small world etc.), then Peter Pan from 11:30-12:30 and then schedule something in frontierland or adventureland for after lunch.