  We present posterior sample-based cosmic microwave background (CMB)
constraints from Planck LFI and WMAP observations derived through global
end-to-end Bayesian processing. We use these samples to study correlations
between CMB, foreground, and instrumental parameters, and we identify a
particularly strong degeneracy between CMB temperature fluctuations and
free-free emission on intermediate angular scales, which is mitigated through
model reduction, masking, and resampling. We compare our posterior-based CMB
results with previous Planck products, and find generally good agreement, but
with higher noise due to exclusion of HFI data. We find a best-fit CMB dipole
amplitude of $3362.7\pm1.4{\mu}K$, in excellent agreement with previous Planck
results. The quoted uncertainty is derived directly from the sampled posterior
distribution, and does not involve any ad hoc contribution for systematic
effects. Similarly, we find a temperature quadrupole amplitude of
$\sigma^{TT}_2=229\pm97{\mu}K^2$, in good agreement with previous results in
terms of the amplitude, but the uncertainty is an order of magnitude larger
than the diagonal Fisher uncertainty. Relatedly, we find lower evidence for a
possible alignment between $\ell = 2$ and $\ell = 3$ than previously reported
due to a much larger scatter in the individual quadrupole coefficients, caused
both by marginalizing over a more complete set of systematic effects, and by
our more conservative analysis mask. For higher multipoles, we find that the
angular temperature power spectrum is generally in good agreement with both
Planck and WMAP. This is the first time the sample-based asymptotically exact
Blackwell-Rao estimator has been successfully established for multipoles up to
$\ell\le600$, and it now accounts for the majority of the cosmologically
important information. Cosmological parameter constraints are presented in a
companion paper. (Abriged)
