Clarification of question 4.4: What would the most ideal combination of methods be to ensure confidence in both size and chemical ID…..and to confirm that the particle is a polymer and not a small molecule that shows the same spectral and/or py-GC/MS fragments as the (oxidized) polymer?

There are likely to be numerous combinations of techniques that can provide reliable quantitative and qualitative information. Whatever combination is employed must demonstrate its accuracy and reproducibility - e.g. via replicate analysis of one or more of standard/certified reference materials, appropriate matrix spikes, and/or successful participation in an interlaboratory comparison.

Microscopy (SEM) (for morphology) + micro-FTIR/Raman. To avoid the issue noted, cross-validate FTIR/Raman spectra with known polymer libraries and degradation profiles and apply thermal analysis to confirm polymeric behavior (e.g., melting point). And, whenever possible, document uncertainty and report confidence levels in ID.

The most reliable way to confirm both particle size and chemical identity, and to distinguish a true polymer particle from a small molecule with similar pyrolysis or oxidized fragments, is to use a combination of complex methods that address morphology as well as surface chemistry, molecular weight, and structural backbone. The workflow typically begins with careful isolation and solvent washing of the particle to remove adsorbed low-molecular-weight contaminants, followed by size and morphology analysis using SEM or TEM (and NTA or DLS for distributions). A particle-scale chemical fingerprints can then be obtained by µ-FTIR and µ-Raman, which can reveal polymer vibrations distinct from sharp small-molecule peaks, while surface analysis by ToF-SIMS and XPS provides fragment ladders and oxidation states consistent with repeating units. Some thermal methods such as TGA-MS/FTIR and py-GC/MS, ideally coupled with thermochemolysis, could add complementary information on decomposition behavior and diagnostic monomer derivatives. Solid-state 13C NMR (CP/MAS) then offers one of the most definitive confirmations, directly showing the polymer backbone environment. If ambiguity remains, controlled depolymerization followed by LC-MS or GC-MS analysis of monomers can provide final confirmation. In practice, a tiered workflow that combines microscopy (for size and morphology), vibrational spectroscopy (for preliminary chemistry), surface and thermal analysis (for decomposition behavior), and at least one high-confidence molecular-weight or structural method (SEC-MALS, MALDI-TOF, or solid-state NMR) gives the greatest confidence that a particle is truly a polymer rather than a misleading small molecule.

To ensure confidence in both size and chemical ID, and to confirm the particle is a polymer not a small molecule, a combination of analytical techniques is needed. 
Raman Spectroscopy has the capability to identify both the size and type of the polymer. 
FTIR spectroscopy provides polymer identification.
SEM or AFM identify shape, size and surface features. 
Pyr-GC/MS is good for chemical confirmation. 
To avoid misidentifying small molecules or additives as polymers, pre-cleaning and matrix removal are essential. Enzymatic and oxidative digestion are good methods to remove the matrix before analysis. 
Cross-validate results with the above techniques are a good way to ensure confidence in both size and ID. Thermal analysis (TGA or DSC) is also a good way to confirm polymer identity. 

First of all a method is needed to verify the presence of particles in the sample. This could be RAMAN or FT-IR but also SEM and TEM are potentially suited. For distinguishing polymers and small molecules in spectra and/or pyr-GC/MS fragments, it is essential to include more than one just characteristic marker fragment and take the ratio between the marker fragments into account. These ratios are fixed for a specific polymer and changes in the ratios are indicative of the presence of other chemicals in the sample - possibly small molecules as well as possibly other ploymers.

Complementary methods to confirm the findings would yield confidence in data quality.  Availability of pure analytical standards of MNPs, internal standards (when mass spec methods are used), proper sample prep and analyte enrichment methods and suitable instrumental methods are ideal combinations.  Mass spec methods can be completed with FTIR or Raman spectroscopy.

This question would be better stated: What would the most ideal combination of methods be to ensure confidence in both morphology and chemical ID?
Raman or FTIR and PY-GC/MS/MS
Py-GC/TOF MS for nontargeted analysis
PY-GC/MS/MS for targeted analysis
There are usually unique pyrolyzates that are the result of molecular recombination during the pyrolysis process of polymers that cannot occur with small molecules. Unfortunately some of the pyrolyzates are in lower abundance than pyrolyzates used for routine identification.  Hence the need for TOF MS or MS/MS identifications for certainty that rules out small molecules as a possible pyrolyzate source.

The most ideal combination will be a chromatography method (SEC or GPC) for size and spectral method (py-GC/MS or NMR). 

I wouldnt change my answer from round 2.